Modern Geography

In the year 1859 there occurred three events which, though not all comparable to one another, yet make the year one of such importance that we may take it as marking the beginning of the distinctively modern period of geographical science. These three events were, first, the deaths of Humboldt and Ritter, two great geographical pioneers who hewed tracks through the tangled jungle of unsystematised geographical facts, and second, the publication of the Origin of Species, by Charles Darwin, a book which supplied the compass which has made further road-making in that same jungle possible.

In other words, as a result of the life-work of the two great geographers named, and of the throwing by Charles Darwin of a new ferment into the mass of contemporary thought, what had been a mere collection of facts began to be a reasoned and ordered science. Both Humboldt and Ritter lived to a great age, so that at the time of their deaths not only was their work done, but there had been time also for their influence to permeate the literature of the subject.

Humboldt was, above all, a great traveller, but he was also a man of science in the largest sense, interested not in one group of facts, but in many. The extent of his knowledge and the breadth of his interests enabled him to observe a vast number of phenomena while his particular genius was manifest in the way in which he correlated these, and considered them in their relation to each other. Though it is true that his influence was most direct in the case of natural history, yet in this respect also he pointed to the future, for the geographers of to-day are indebted to the naturalists for some of their finest generalisations.

Ritter was a great teacher, the prototype of those who alike by their personal influence and by their books have enriched geographical science within the last fifty years. He had not Humboldt's breadth of knowledge and interest, but in the stress which he laid upon the earth as above all interesting in that it is the field of the activity of man, he emphasised an aspect of the subject in which perhaps the most interesting modern developments have taken place.

Darwin had a twofold effect upon the progress of geography. In the first place, in his detailed work, e. g. in connection with coral reefs, and with the distribution of animals, and less directly in his investigation of the part played by earthworms in the formation of soil, he himself added to geographical knowledge. But he did much more than this. The doctrine of evolution which he made common property has had and is having an enormous effect upon geographical science, both directly and indirectly.

As is well known, in connection with his own theory of the cause of evolution, Darwin laid great stress upon the "Struggle for Existence." But he himself expressly stated that he used the term in a "large and metaphorical sense," a sense which in popular language it has tended to lose. From the geographer's standpoint, therefore, it is better to say that Darwin's work has added a new interest to the study of interrelations. Humboldt, as we have indicated, was greatly interested in such subjects as the connection between the climate of a region and the vegetation, between the activities of man in a particular region and the physical conditions, and so on. But Darwin added a new interest to such studies. For example, it is a curious fact that desert plants have often spiny leaves, long roots, and so forth, and it is interesting to note how these peculiarities fit the plants for life in an arid climate. But when Darwin showed that there was evidence that the physical conditions of the desert gave rise to certain types of vegetation, it became worth while to study both the physical conditions and the characters of the plants in much greater detail than before.

If we simply lay it down as an axiom that, e. g. cactuses live in deserts, the fact has only a moderate interest, but when we find that almost any natural group of plants, if exposed through long ages to gradually increasing conditions of drought, will produce "cactus" types, then the whole subject acquires new importance. This illustration may serve to suggest what Darwin has done for geography.

He showed that there is a delicately adjusted balance between organisms and their surroundings, taken in their widest sense. But geology proves that through the ages there have been constant, if slight, changes in the physical conditions, and the effort of the organisms to readjust the balance thus disturbed has led to evolution. Thus to some extent at least the characters of organisms can be explained by the nature of their surroundings. A further interest is added by the fact that in this respect human societies and settlements can be shown to behave like organisms. Therefore we can hope to explain at least partially the manifold differences in man and his societies in different parts of the globe by the minor differences in physical conditions. In other words, the doctrine of evolution has added a unifying and co-ordinating principle which has not only prevented geography from being crushed by the enormous recent increase in known facts, but has also for the first time raised it to the level of a science.

This addition of a co-ordinating principle may be said to be the direct effect of the publication of the Origin of Species, but there has been an indirect effect almost as important. The principles enunciated in that book had a stimulating effect, not upon one science only but upon every department of thought. Phenomena of no importance suddenly became interesting, and the result of this interest was an enormous addition to known facts. Not only has research been stimulated in every direction, but as this research has been largely directed by the desire to discover the interrelation of phenomena, we find that many of the old barriers between the sciences are breaking down.

The botanists are no longer content to study the facts of plant distribution; they now want to be able to give reasons for particular distributions. Therefore they must seek the aid of the meteorologists to explain differences of climate; of the physical geographer to make clear the effects of relief, of differences of soil, and of drainage; of the cartographer to represent the facts which emerge from their surveys, and so on. The physician must now seek the assistance of the zoologist before he can deal adequately with tropical disease, and the zoologist must have the help of the physical geographer before he can give adequate aid. The result is that in all directions geography is being enriched by facts brought from the collateral sciences, while at the same time its position as a central unifying science is becoming more and more established; as a science which can deal with all these varied facts, but deal with them from a standpoint peculiarly its own.

At the present time, geography may be compared to one of Rodin's statues in which we see a beautiful figure as it were struggling to escape from the marble in which it is imprisoned. So the geography of to-day is in the act of escaping from the matrix of mere facts in which it has been too long imprisoned. It is now displaying itself as a great unity in the making of which all the sciences have played their part.

But even in this general survey of recent developments two other sets of facts must be touched upon. We have given fifty years as the period within which most of what is distinctively modern in geography has developed. It must not be forgotten that within the same period there has been a remarkable renewal of interest in geographical exploration. Roughly speaking, within this period Africa has ceased to be an unknown continent; the innermost recesses of Asia have been largely explored; the Arctic and Antarctic areas have yielded many, though by no means the whole, of their secrets; a great deal of exploration has been done in America as a whole, as well as much detailed survey work in the United States and Canada; the oceans have been investigated by successive series of expeditions. Generally it may be said that in its broad outlines our knowledge of the world has been completed, so that geographical science is free to pass from the mere collection of raw material to the higher task of arranging, classifying, and making deductions, as well as to the more detailed surveys which are still necessary.

The other point of interest is that the last fifty years have seen an enormous increase in the facilities for travel, a fact which has led to a great increase in the number of people to whom geography appeals. The decade between 1830 and 1840 saw the beginning of two great series of guide-books, Murray's Handbooks and Baedeker's Guides, whose importance for the travelling public can hardly be over-estimated. The first "Baedeker" was a little guide to the Rhine, and since it was first published this firm of publishers has not only extended its field of operations over nearly the whole world, but has issued a constant stream of new editions, which for the most frequented tourist regions are practically annual. That great tourist agency whose name is now a household word began operations in the early forties, and like the firm of Baedeker has now taken the world as its sphere of action. We may say, then, that during the course of the nineteenth century, travel, previously a pastime of the rich, was brought within the reach of very moderate purses. This democratisation of travel is still going on, and in certain recent visits of British working men to Germany and elsewhere we may perhaps see the beginning of a process which will eventually bring some amount of journeying abroad within the reach of all.

As yet the effect upon geography of this increase in travelling has been chiefly to enhance popular interest in the science, rather than to enrich it, for the vast majority of "popular" travel books have added little, if anything, to the sum total of knowledge. But this is partly because geographical teaching has hitherto been badly organised, and the greater number of travellers have started on their journeys without having been taught what to observe or how to observe. There are already indications that this condition of affairs is passing away, and that the traveller of the future will start better equipped, and will demand in his guide-books a new point of view. Starting from a higher level he will bring back more from his travels.

Meantime it should be noted that some knowledge of the generalisations laid down by geographers during the course of the last half century adds enormously to the interest of travel, both at home and abroad, and that for this reason, if for no other, geography is worth study by all.

In the following chapters we shall look, so far as possible, at those aspects of the subject which make the widest appeal, and which are best fitted to enable the ordinary man to understand his surroundings, whatever they may be, and so aid him in that delicate task of adjustment which, consciously or unconsciously, is the task of every living thing. As limitations of space involve a similar limitation of subject-matter, it has been thought best to lay most stress upon the conditions which prevail in Europe and North America, the areas which have been most thoroughly studied. Europe has the special interest that it has given origin to the type of civilisation which has most profoundly modified the earth's surface. This limitation cannot, however, be made rigid, for it is of the essence of the modern standpoint that no area can be understood without reference to the world at large. The geography of Europe no less than of North America is determined by the position of the respective continents on the surface of the globe, and cannot be understood without a consideration of this position and its implications. The standpoint adopted here is frankly anthropological, that is, the world is considered as the home of man, its physical peculiarities being regarded as interesting chiefly in their relation to man and his activities.

Finally, we may note that the development of the subject within recent years has been such that it is quite impossible, even within the limitations already laid down, to give a complete survey of the subject. All that will be attempted, therefore, is to suggest some of the lines along which research is proceeding most actively at the present time, special stress being laid upon those aspects of the subject which are not as yet fully treated in the smaller text-books. The list of books of reference at the end will, it is hoped, enable those interested to fill in the blanks which such a scheme necessarily leaves.

* * *

It is not necessary here to consider the various formal definitions of geography which have been proposed in the last few years. As is only natural with a developing subject, much discussion has taken place as to the exact limits of its field of action, and many definitions have been proposed with the object of setting forth these limits as clearly as possible. But it is sufficient for our purpose to note that geography deals with the surface-relief of the earth, and with the influence which that relief exercises upon the distribution of other phenomena, and especially upon the life of man.

Before we proceed to study detailed problems, then, it is obviously necessary to look at some general points connected with the relief of the earth's surface and its causes.

In the words of the physical geographer, the earth's surface consists of the solid crust, or lithosphere, of the mass of water forming the seas and oceans and constituting the hydrosphere, and of that envelope of gas which we call the atmosphere. Considered separately, each of these is the concern of special sciences, and not of the geographer proper. His business it is to take the facts furnished by the meteorologists, the physicists, the geologists, and so forth, and with these facts in hand to proceed to consider the effect of the interaction of earth and water and air in a way which the separate sciences cannot do. We must further note that it is the interactions of these three which make the earth a possible home for life as we know it, and it is these interactions therefore which influence the distribution of life on the surface of the globe.

There may have been a period when the crust of the earth was clothed in a uniform sheet of water, just as the globe is now enveloped in a complete covering of air, but at present, as through the long ages of geological time, the lithosphere consists of elevations and hollows, and it is in the hollows that the water accumulates, so that we can distinguish between the dry land and the ocean beds. Both chemically and physically the fluid hydrosphere differs markedly from the solid lithosphere, and it is, above all, the physical differences which are of supreme importance to the geographer. Because of them sea and land respond differently to the stream of solar energy which pours down upon our globe, and it is this different response which is the predominating factor in the production of the different climates, which again determine in its main outlines the distribution of living organisms.

This being so, it is clear that it is of great importance to the geographer to know exactly the distribution of land and water over the surface of the earth. As the North Polar regions are still inadequately known, and the South Polar regions hardly known at all, we cannot as yet determine exactly this distribution, but any globe will show that land and ocean are very unequally distributed. The great land masses cluster round the North Pole, while the southern hemisphere consists largely of water. We thus have a land hemisphere and a water one. According to recent calculations the oceans occupy some 72 per cent. of the entire surface of the globe, leaving only 28 per cent. of land. But while in the northern hemisphere there is about one and a half times more water than land, in the southern there is about six times more water, both figures being liable to error, as indicated above, owing to our uncertainty as to the land and water of the Polar zones.

This distribution is of great importance in connection with certain theories as to the actual plan of the earth, but this is a difficult subject which need not concern us here. It is discussed in Prof. J. W. Gregory's volume on The Making of the Earth. More interesting is the effect which the arrangement of land and water has had upon that part of the life of the earth which was evolved in late geological time. Though the geographer for convenience' sake recognizes three separate continents in the Old World-Europe, Asia and Africa-yet these form practically one land mass, which in its turn approaches America very nearly at Bering Straits, and, less nearly, in the North Atlantic through the intervention of the British Isles, the Faeroes, Iceland, Greenland, etc. The centre of this land mass lies in Europe, a point not without its importance.

In this great land mass of the northern hemisphere life has reached its highest degree of development, both as regards animal form and as regards human societies. It was in the northern hemisphere that the highest mammals, the placentals, arose. There are many remarkable resemblances between the faunas of Europe, of Asia and of Africa, and a similar, if less marked, resemblance between those of North America on the one hand and of Europe and temperate Asia on the other. On the other hand, the two great land masses which occur in the southern hemisphere, South America and Australia, show very marked differences in their fauna, both from each other and from the northern land masses, and in both cases the fauna has a primitive aspect, which is best marked in Australia.

When we come to consider man, somewhat similar conditions present themselves. The great civilisations developed in the land mass of the Old World, though the waterless desert of the Sahara cut off much of Africa from participation in them. America developed a relatively high civilisation of its own, but as the icefields and ice-pack of the north formed a greater barrier to the migrations of man than to those of the northern animals, this American civilisation was for long cut off from that of the Old World, and when free communication became possible, it went down before that of the eastern world.

We must connect these facts directly with the peculiar distribution of land and water in the northern hemisphere, which made free intercourse possible, alike for the land animals and for man. The importance of this intercourse may be suggested in a few words. When a group of organisms is limited, from whatever cause, to a particular zone of the earth's surface, the members of the group tend to acquire characters fitting them for this restricted area. But if the area is open, constantly or periodically, to incursions of organisms from adjacent areas, then, with the widening of the environment, and the greater intensity of the struggle for existence, evolution is quickened and new characters appear. The men of the Eurasian continent learnt, on the fierce battle-grounds of that continent, lessons which enabled them to conquer without difficulty the more isolated human groups of the southern hemisphere. The fact that they took south with them the mammals of the north, who also have thriven at the expense of the native forms, shows that the hold of the southern animals upon their habitat was no less precarious than that of man himself.

One other point is worth notice in connection with the distribution of land and water over the surface of the globe. We have seen that the northern hemisphere is the region where organic evolution has been most marked. It is, as it were, a great biological laboratory. On the other hand, in the southern hemisphere, which has fewer land masses to interfere with the circulation of the atmosphere, many physical phenomena occur in a more marked and orderly fashion than to the north. The westerly winds of the south blow with a force and a constancy which makes it impossible to compare them with the more variable westerlies of the north. Even the ocean currents of the south seem to show more constancy than those of the north. If the northern hemisphere is a great biological laboratory, the southern may be described as a physical one, and one of the great interests of the further exploration of the Antarctic is that it will probably cast light upon some important meteorological problems. (See Dr. W. S. Bruce's volume on Polar Exploration.)

The distribution of land and water, with all its effects on climate and on the distribution of life, is, as we have seen, caused by the main features of the relief of the earth, by the existence of vast depressions in which the water accumulates, and of relative elevations from which it flows. But the minor details of relief, hill and valley, ocean depth and continental shelf, are also important, and exercise a very marked effect upon distribution. They therefore demand in their turn some consideration.

Taking first the prime distinction between land surface and ocean floor, we note that the two differ from one another markedly, alike in their characteristics and in the conditions to which they are exposed. The land is subjected to constantly varying conditions: to the alternation of day and night, and to the changes of the seasons, with corresponding variations in temperature; to the fluctuations of the weather; to running water, and so forth. In the great ocean depths at least, on the other hand, the conditions are remarkably uniform. Neither diurnal nor seasonal changes have here any effect; the temperature seems to fluctuate but little; the water is almost still. This uniformity of physical conditions is reflected in the uniformity of the surface over wide areas. While the land surface shows marked irregularities, the ocean floor has a monotonous character, with more gentle outlines.

In its most general form the characters of the sea bottom may be briefly stated. Round the great land masses there is an area of relatively shallow water, which is sometimes only a few miles wide, and at other times extends outwards for hundreds of miles. This region is the Continental Shelf, and its seaward boundary for convenience' sake is taken at a depth of 100 fathoms, or 600 feet. Within this zone the influence of the land is still felt, and some of the characters of land surfaces appear. Thus we sometimes find that river valleys are prolonged outwards over the Continental Shelf, giving a markedly irregular appearance to the ocean floor. The British Islands lie upon a Continental Shelf of this kind, and this is one of our reasons for knowing that they are really only a part of the continent of Europe, separated from it by a slight depression.

The Continental Shelf slopes away from the land gently, and is widest where it fringes low continents, and narrowest where mountains approach the coast. Over it is spread the waste of the land, the coarser lying near the shore-line, the finer extending outwards to the steep seaward slope. This rapid slope leads down to the more or less uniform ocean plateau, whose surface is broken by the great ocean abysses, the greatest of which has a depth of about six miles. Relative but not absolute uniformity thus characterises all that part of the ocean floor which lies below about 100 fathoms.

Again, though the ocean floor is doubtless being slowly raised by the deposition upon it of the oceanic oozes, yet it is also true that as compared with the land surface it displays great constancy. While the land surface is constantly changing owing to the varying forces which act upon it, the floor of the ocean can vary but little from age to age, unless it is acted upon by the internal forces of the earth.

Turn now to the land. We note at once the two characters of marked irregularity of surface, and of changeableness. The changeableness is due to the forces of erosion which act upon the surface, and of these forces the most important to the geographer is running water. It is running water, aided by other agents, which carves the land into hill and valley, which produces gorge and lake, only ultimately to fill up the lake and plane away the gorge. It is running water which spreads out on the lower ground the waste of the higher, and thus prepares the way for the operations of man.

The result of the long-continued action of the varied forces of erosion must necessarily be to reduce the surface to an almost level condition. The denuding agents first produce irregularities and then finally remove these, until the whole surface is once again almost level. The whole globe would thus be reduced to the condition of a plain were it not for the intervention of the internal forces which raise up the surface anew into folds, or which produce volcanoes and outbursts of molten rock.

This constantly repeated series of changes may be said to be chiefly the concern of the geologist, especially as it is a series which has repeated itself in all time. But it is to be noted that at various parts of the surface of the globe at the present time every stage in the process occurs, and everywhere the question whether a particular land area has been exposed for a relatively long or for a relatively short period to the forces of erosion, has a profound influence upon life. It is therefore important for the geographer to be able to recognise the different stages. This he cannot hope to do without some detailed knowledge of the effects of erosion.

Theoretically every land surface elevated above sea-level should pass through what has been called a cycle of erosion. There should be a period when the active forces are working upon a surface as yet but little modified; this is the period called by analogy youth. At a later stage the drainage has been well established, and the rivers run in broad valleys, from which lakes and waterfalls have largely disappeared. To this condition the term mature has been applied. At a still later stage the land surface has been so worn by the eroding forces that the whole process of erosion is slackened, and an uplift must occur before the erosive forces regain their lost strength. This is the so-called "cycle of normal erosion," but it is constantly liable to variations due to local crust movements, to changes in climate, and to local conditions, though at the same time the distinction of the various stages has value for the geographer because of their varying effects upon human life. It is necessary for us, therefore, to consider how the different stages may be recognized, and how the forces of erosion act.

Let us begin our study of erosion by a general survey of the striking features of the earth's surface at the present day. We know that at various parts of the surface there rise lofty mountain chains, whose summits are often permanently snow-clad, and which, from the sharpness of their forms and from the masses of rock rubbish which are accumulating round them, have obviously only been exposed for a geologically short period to the action of the atmosphere and of running water. When examined such mountain chains are all found to have the same peculiarities of internal form, the rocks composing them being elaborately folded and fractured. Careful investigation has convinced geologists that all the existing great chains owe their origin to a series of earth movements which occurred in the period called Tertiary, that is, in the third of the great geological periods, the one immediately preceding that in which we live.

These lofty mountain chains of Tertiary origin are most familiar in the great series of folds which appear at the surface to form the Pyrenees, the Alps, the Caucasus and the Himalayas, but the Atlas Mountains belong to the same series, as does also that great mountain chain which, under various names, runs down the western coast of the American continent.

Fig. 1.-The main points in regard to the structure of Europe. The shaded areas (1) are regions of ancient rocks, much folded and crumpled, which once formed mountain regions but are now mostly worn down to uplands. The lines (2) show the regions affected by Tertiary folding, largely occupied by mountain chains. The unshaded areas are mostly plains and basins, only slightly affected by folding, and made up of rocks which are often almost horizontal.

As already indicated, these areas are recognised not only by the fact that there appear at the surface a great number of peaks forming a mountain chain, but also by the internal structure, the characteristically complex folding of the rocks. Now outside of these recently elevated areas in, for example, the continent of Europe, we find two conditions. On the one hand, there are regions of upland type but with rounded and smoothed forms, which are sometimes almost reduced to the condition of a plain. Such regions occur in Ireland, in the west of Great Britain generally, in Brittany, in the central plateau of France, in the Ardennes, in Bohemia, in the central plateau of Spain, in Scandinavia, and so forth. Between these relatively elevated areas we have plains and low-lying river basins, such as the London basin, the Paris basin, and so on. When the rocks are examined in both cases it is found that in the basins and plains the rocks, as a general rule, are only slightly inclined, while in the uplands and plateaux there are obvious remnants of folding, and the rocks are of ancient types, not relatively modern like those of the Alps, Himalayas, etc. (see fig. 1).

If, then, the existing mountain chains show complex folding in their constituent rocks, and though geologically but of yesterday have been already deeply affected by the denuding agents, must we not suppose that the folded and contorted uplands of Europe and elsewhere are the last remnants of very ancient mountain chains? It is they which form the framework of the continents, and by their wear and tear the low grounds have been formed, owing to the filling in of the great gulfs which ran between the old mountain chains.

We may elaborate a little further this very interesting subject. Let us first note that the geologists group the rocks composing the earth's crust into three great divisions. We have, first, the Primary rocks, which are the oldest, and include as their most generally interesting member the Carboniferous rocks, with their coal-bearing beds, so important in the modern industrial world. Second, we have the Secondary beds, the most interesting members of which is the Chalk, so well-developed in parts of England and France. Finally, the Tertiary series includes the rocks of the period immediately preceding that in which the first undoubted remains of man occur.

Each of these periods was of enormous length, and the labours of successive generations of geologists have brought to light, at least in broad outline, the general appearance of the globe in so far as affected by the distribution of land and water, and the main earth movements, in each separate period. Thus we know that during that long period of time which is included in the Primary epoch, very extensive earth movements, resulting in extensive folding and mountain formation, took place. The geologists distinguish no less than three separate periods of folding in Primary times. It is not necessary for us to consider these in detail; their total result was to produce the mountain regions whose worn-down stumps now form those uplands which we have described in Europe. But they do not occur in Europe alone. That vast and relatively infertile area in Eastern Canada which geologists call the Canadian Shield is a region of very old rocks, once folded into a mountain region, but long since worn down to an upland. In the eastern United States that long, but interrupted, range of hills, which, under various names, runs from the mouth of the St. Lawrence to Alabama and Georgia, and partially shuts the seaboard off from the prairies and plains beyond, is a region where the folding is still well marked, in spite of long denudation.

The Secondary period seems to have been one in which comparatively little folding took place, while, as already indicated, the Tertiary was one in which there was enormous folding in almost all parts of the globe, the result being the appearance at the surface of the great mountain chains of the present day. The structure of these chains makes them relatively unstable, and the forces of erosion are now acting upon them with extraordinary activity, beginning that process of wearing down which has reduced their prototypes of the Primary period to mere remnants of their former greatness.

Extensive as the Tertiary folding was, however, it left great areas unaffected, or but slightly affected, and such areas form plains or basins, where the rocks are but slightly tilted, or show a very simple form of folding. In Europe such slightly modified rocks occur, e. g. in the Paris basin, and in the fertile plains of south-eastern England.

In the United States beds of a similar character occur right over the great plains, filling what seems once to have been a great gulf between the old highlands to the east and the towering modern mountain chains of the west.

It must be realised that this is only a very summary and partial account of a difficult and complicated problem; but from the standpoint of pure geography it seems desirable to distinguish between those remnants of ancient mountains which form the backbone of the continents, the recently elevated mountain chains where enormously rapid erosion is taking place, and the largely unmodified rocks which often form fertile plains.

Let us next proceed to consider how the eroding agents act upon the surface of the land as soon as it is exposed. We may begin with the effect of running water upon a recently exposed surface, e. g. upon land slowly emerging above sea-level, or even with the effect of heavy rain upon sloping ground unprotected by a covering of vegetation. Alike in the one case and in the other the first effect is the formation of a number of shallow rills, which at first run parallel to one another. Sooner or later, however, these parallel channels tend to converge, and a torrent is formed such as may be seen in any mountain region.

Fig. 2.-An ideal profile of a mature river (AC), showing the increase in the slope towards the source. The dotted line BC shows an earlier stage, when there are smooth reaches and rapid reaches with waterfalls, etc. Note that progressive erosion causes the source to retreat (i. e. from B to A).

Fig. 3.-An actual profile of the Loire. It will be noted that the Loire is a mature river, its profile nearly coinciding with the "ideal" condition. (After de Martonne.)

Such a torrent consists of three often well-defined parts. First we have the numerous tiny rills which collect together to form what the French physiographers call a receiving basin (bassin de réception); then there is the stream proper forming a canal which drains the basin, while below, where the torrent debouches on the low ground, we find that it spreads out fanwise and throws down its load of débris to form a cone (c?ne de déjection). The torrent therefore already imitates a full-grown river, with its threefold division into mountain track, valley track, and plain track. It further illustrates the twofold work of the river, that of erosion and deposition.

Observation on an unprotected surface after a heavy rainfall will illustrate another point which is of much interest in connection with the work of rivers. This is that the water has most excavating power, not, as might be supposed, in the collecting basin, but in the valley region, where the slope is still great, where the volume of the water is at its maximum, and where it has acquired a load of débris by means of which it carves out its bed. The excavation of the bed therefore proceeds from below upwards towards the collecting basin. The result is that the slope of the valley floor diminishes as we pass from the upper region to the lower, owing to the levelling effect of erosion. The process of levelling down cannot be carried beyond a certain point, the so-called base level of erosion, which in a lateral stream is determined by the point of junction with the main stream, and in a main stream by the point which marks sea or lake level, for obviously no point in the river valley can be worn down much below its mouth.

When the work of a river is completed, the line which marks the profile of its bed should have a gentle and continuous slope downwards to base level. The existence of irregularities, of breaks in the smoothness of the slope, means that the work of excavation has not proceeded far, that the river is young. But it is not necessary to proceed to the laborious drawing of a profile in order to determine the extent to which the process of excavation has been carried. The existence of rapids, of waterfalls, the alternation of swift and slow-flowing reaches are all proofs that it has not been carried far. In short, if a river is navigable, the navigable reach at least is mature; if it is capable of furnishing power, that region at least is youthful. If, as sometimes happens, the middle course is navigable and slow-flowing, and the lower course broken by rapids and falls, then the probability is that earth movements have occurred, so that the two regions are of different age. This is a condition which occurs relatively often in the case of large rivers.

One other point is worth notice, because it illustrates another way in which the analogy of youth and maturity holds good. The youthful river, with its interrupted slope, its lakes and falls, does not permit the water to flow off with the same regularity as the mature river with its smoothed outlines. The mature river is thus a more perfect instrument of drainage.

It is not necessary for our purpose to consider in detail the characteristic forms of river erosion. It may be sufficient to notice that rapids and waterfalls are due to the varying hardness of the rocks forming the bed of the river, and that the normal course of events is the transition from waterfall to rapid, and from rapid to stream flowing quietly at the bottom of a rocky gorge. Long gorges or canyons tend to occur in regions where river erosion is not greatly assisted by the other eroding agents. As a general rule, as the river cuts its way down, the other agents cut back the walls so exposed, so that a wide valley is formed.

But a river does not only eat out its bed in its valley track. A necessary consequence of this erosion is that it is also able to eat back the slope on which it is rising, as a result of the smoothing out of the curves of its bed, so that its source retreats further and further into the mountain. In regions of abundant rainfall every slope is abundantly supplied with streams, and therefore those streams which cut back their region of origin most rapidly will necessarily encroach upon their neighbours' territory. They therefore tend to tap some of the tributaries of the other streams, a phenomenon which has sometimes considerable human importance, and has been extensively studied of late years under the name of river-capture.

Some examples may serve to make the phenomenon clear. Every one who has travelled up the Rhone valley in Switzerland has noted the enormous number of lateral streams, of all sizes, which tumble down the mountain sides into the Rhone. These streams on, e. g., the south side, are, roughly speaking, parallel to each other, and to a large extent enter the main stream independently. That is, for the most part they are very youthful streams. In some cases, however, e. g. in the case of the Dranse and the Visp, the drainage is of a more advanced character, and we find a large stream with tributaries of considerable size as distinct from mere torrents. A glance at any great river system on the map, e. g. the Mississippi, the Amazon, etc., will show that the condition of a great stream with many tributaries is normal in a district where the drainage is of the developed type. How are the two conditions, that of numerous parallel mountain torrents and that of a great river system, related to one another? There is no doubt that capture, the encroachment of one stream upon the territory of another, has played an important part in the process.

Fig. 4.-Sketch-map to illustrate river-capture.

A very simple example of this widespread phenomenon may be taken in illustration. The accompanying sketch-map, drawn by Mr. Lionel Hinxman, shows part of the course of the River Feshie, one of the tributaries of the Spey, and part of the Geldie Burn, one of the tributaries of the Aberdeenshire Dee. It will be noted that the Feshie shows a very curious bend, or elbow. Mr. Hinxman points out that this curious condition can be explained on the supposition that the River Eidart, shown on the map to the north of the bend, once formed the headwaters of the Feshie, which cut its valley back until it captured the headwaters of the Geldie, and thus brought water which formerly flowed into the Dee into the Spey valley. The boundary between the two counties of Aberdeen and Inverness is shown on the map by a dotted line, and it is seen that the march follows the watershed, which between the present Geldie and the bend on the Feshie is very low. Formerly, however, this watershed lay much further to the west, and its shifting is due to the capture.

A careful study of large scale maps will show many examples of similar river-capture, some old and some recent. A sharp bend, the so-called elbow of capture, on a river in close proximity to another stream affords in itself a certain presumption that capture has taken place, though this presumption can only be verified by study on the spot.

It may be noted that before the capture is finally accomplished there may be an intermediate stage when the water has the choice of two channels, both of which may be utilised in a time of flood. A very curious case is that of the Casiquiare, a river in South America which connects together the two systems of the Amazon and the Orinoco, while another is the connection recently discovered by Captain Lenfant, a French explorer, between the systems of the Shari and the Niger in Africa. Such conditions are obviously unstable, for one stream must sooner or later predominate over the other, and deprive it even of flood water.

Another example may help to explain the evolution of a complex river system with many tributaries. A glance at the map of England (see diagram) shows that while the rivers of Northumberland and Durham flow independently into the sea, those of Yorkshire are united into a characteristic bunch, and all reach the ocean by means of the Humber. This estuary breaks through the high ground formed by the Wolds of Yorkshire and Lincolnshire, which consists of hard rock. At one time it is probable that the rivers of Yorkshire entered the sea separately, while the other great factor of the Humber, the Trent, mingled its waters with the present Witham. At this time the weathering of the land surface had not reached its present stage so the land would lie higher. In what is now the vale of York the rocks are much softer than where the Wolds now stand, and the present Ouse, which was at first a longitudinal tributary of a transverse stream, eating its way back through these soft rocks, tapped successively the streams flowing eastwards from the Pennines, and with the help of the abundant water so obtained was enabled to cut out the wide estuary of the Humber.

Fig. 5.-Sketch-map of northern England, to show the position of the Tyne and Aire Gaps, and the peculiar character of the rivers of Yorkshire. The black areas are heights above 600 feet.

One other important point in connection with river-capture has been already suggested in the account given of the Feshie. In the little sketch-map we see clearly the shift of the watershed to the east. The ultimate cause of this shift is doubtless the fact that in Great Britain the rainfall diminishes to the east, so that, generally speaking, the westerly streams have more erosive power than the easterly. But the special interest of the case is simply that it may serve to suggest a fact not at first sight obvious, which is that water-partings are excessively unstable features. One set of streams is continually striving to encroach upon the others, and by capturing their headwaters to reduce their erosive power. A very striking example of capture on the large scale is seen in southern Patagonia, where the water-parting does not lie near the summit of the chain of the Andes, as might be expected, but considerably to the east, the western streams (or glaciers) having captured all the headwaters of the eastern streams, which lie in a region of much lower rainfall.

The net result is that running water not only scours valleys in the sides of mountain chains, but also, sooner or later, wears away the crest itself, and with the assistance of the other agents of denudation tends to reduce the mountains to plains-or at least "peneplains." The deduction is, of course, old enough, but the recent emphasis placed upon river-capture helps us to realise it, showing us the actual "shift of the divide," or, in other words, the wearing down of the summit levels.

This is a theoretical matter, but there is another point which has practical significance. Referring once again to the sketch-map on p. 43, we note that just at the sharp bend in the Feshie, that is, at the elbow of capture, there is a narrow region, crossed by the boundary line, which was once traversed by the headwaters of the Geldie, but is now a dry valley. Such "gaps," as they are called, are present where recent capture has occurred, and where they occur in hilly country they sometimes form useful passes, permitting the construction of an easy road across the hills. A good example is the Aire Gap (see fig. 5) in the Pennine range of Great Britain, apparently connected with the fact that the Ribble has captured the headwaters of the Aire. Another interesting example is the so-called Tyne Gap, that breach in the Pennines which occurs near the present head of the South Tyne; it was traversed by the Roman wall, and is now crossed by the road and the railway from Newcastle to Carlisle.

As we shall see, ice appears to have this power of cutting passes through mountain chains to a much greater extent than running water; but here, as in many other respects, there does not appear to be a sharp breach between the action of the two.

* * *

In the last chapter we have spoken of the moulding of the surface of the earth by means of running water and the agents summed up in the term "weathering." The process is sometimes called "normal erosion," to distinguish it from that other form of surface moulding in which ice and frost play a prominent part. At the present time ice, in the form of ice-sheets or glaciers, is confined to relatively small areas of the globe, so that we are justified in regarding its action as exceptional when compared with the work of running water.

It is, however, well known that this limitation of the field of action of ice is very recent, and that during a period which geologically is only yesterday, a much greater part of the surface than at present was ice-clad.

In point of fact, much of Europe, especially the northern parts and those regions which lie close to the lofty mountain chains, much of North America, and, probably, considerable parts of the southern hemisphere, were subjected to the action of ice so recently that the processes of normal erosion have not had time to obliterate, hardly even to blur, the tracks which the ice left.

The results of the great extension of ice action in that period which geologists call Pleistocene were twofold. In the first place, as the result of the presence of the ice-sheet, we have vast accumulations of débris spread over the lower grounds. These accumulations sometimes form great sheets of boulder clay; sometimes they are collected into the curious sandy and gravelly mounds called kames which in parts of, e. g. Scotland, have a great extension; sometimes they have formed great heaps of material at the entrances of valleys. Again, these deposits have sometimes blocked valleys and so formed lakes, and they have supplied the post-glacial rivers with a vast amount of material which has been used to scour out the river-beds, and has been often re-sorted and re-arranged by running water.

Secondly, the fact that the northern region and the high grounds further south, in both Europe and North America, have been recently clad in ice is associated with many peculiarities of surface form, some of which have exercised a marked influence on human settlements and ways of communication.

These peculiarities of surface moulding have been the object of singularly detailed study in late years, and from this detailed study many interesting facts have emerged. It may be well to state at once that this study has been largely stimulated by the fact that there is at present a great want of unanimity of opinion as to the exact cause of these peculiarities of form. According to one school ice is a more powerful eroding agent than water; according to another its action is largely conservative, and its power of erosion is slight as compared with that of water.

The beginnings of a possible solution of the problem are perhaps to be seen in the suggestions of those who seek the causes of the peculiar features of glaciated regions in the way in which running water works when it is controlled and modified by the existence of ice; but we must admit that, on the whole, the conflict is still hot and many members of the opposing schools will have no compromise.

To the geographer, however, the very fierceness of the controversy has been useful. The question as to the exact part played respectively by water and by ice in surface moulding is really a question for the geologist. It is, however, of great importance to the geographer that recently glaciated surfaces should be studied from every point of view, for from this detailed study are emerging many important generalisations. We shall, therefore, in this chapter only touch very lightly upon the actual points in dispute, but shall lay stress upon the interesting facts admitted by both parties.

When the conception of a just-vanished period of great glaciation was being established by the labours of many geologists, stress was naturally laid upon the obvious resemblances between parts of, e. g. Scotland and Wales, and those parts of the Alps which have been exposed by the retreat of the existing glaciers. Thus we find that most of the text-books emphasise the occurrence of perched blocks, of erratics, i. e. of blocks of rock which must have been carried from a distance, of the phenomenon of crag and tail, of giants' kettles, and so on. All these are of more geological than geographical importance; they do not in themselves greatly affect the distribution of other phenomena over the surface. We shall not, therefore, stop to consider them in detail. It is otherwise with those indications of recent glaciation which have been studied within the last few years, and they demand the geographer's most careful consideration.

The most active discussion has taken place in regard to the peculiar features of the valleys in recently-glaciated districts, and we shall discuss especially this point.

We have already described the general features presented by valleys which owe their origin to running water. In such valleys, as we have seen, the longer the forces work the more nearly is the valley floor reduced to an even slope, whose angle decreases in passing from the mountain to the plain track. In the ordinary river valley the shape of the valley approximates to that of a V, that is, the valley narrows downwards, the river occupying the narrowest region.

Again, as a general rule there is no great difference of level between the tributary valleys-at least at their extremities-and the main valley, that is, there is no sharp discordance between the two. While, however, the "mature" river valley shows a gentle, continuous slope, we usually find that "young" rivers, at least in their mountain track, show an alternation of plain and gorge, which is very easily observed in any hilly region.

In other words, we find that, owing to the inclination of the rocks, or to their varying hardness, or to other causes, particular reaches are less easily eroded than others. These form waterfalls, which ultimately, as we have seen, give place to gorges. Beyond the waterfall the diminishing slope checks the rapidity of flow, and the stream tends to widen out, and also to throw down its load of débris, so that an alluvial plain may be formed.

One other character of an ordinary river valley may be noted. It heads, as we have seen, in a collecting basin, which receives the surface runnels and the outflows of the springs which form the beginning of the river.

Let us now turn to the valleys in a recently glaciated country. We omit any description of existing glaciers; these will be found described in the volume on the Alps, and further, photography and the picture postcard have rendered the main features of a glacier familiar to every one. Almost every large railway station now shows fine coloured photographs of some of the important Swiss glaciers.

Taking, then, a valley known to have been occupied by a Pleistocene glacier, we find the following features. As contrasted with an ordinary river valley, the glacial valley is usually flat-bottomed, a condition described as U-shaped to point the contrast with the river valley. Examples in Great Britain and elsewhere are frequent, but some of the Alpine valleys show the phenomenon in a very striking form. Two good examples are the Aar valley at Meiringen, and the Lauterbrunnen valley at the village of the same name. Both have been rendered more or less familiar by constant photographing (see fig. 7).

The reason why they have been so much photographed leads us to consider another peculiarity of the glaciated valley. In both the cases named a steep cliff wall rises from either side of the broad, flat valley floor, and from the summit of this cliff the lateral streams leap into the main valley by often superb waterfalls. This is a very important feature of glaciated valleys-the fact that their tributaries are markedly discordant, that is, that there is marked difference of level between the beds of the side and main streams.

Because the side valleys lie high above the main they are said to "hang," and are called hanging valleys, while the main valley is said to be over-deepened. The rocky height over which the water springs may be called the junction step, as an attempt to translate the French term gradin de confluence which is applied to it.

Incidentally we may note that in the Alps the junction step is of great human importance. Its presence gives the water the power which is used in lighting the Alpine villages with electricity, and in driving the trains which often carry the tourist to those villages. In the French and Italian Alps especially, the power is being more and more used to supply the motive force for various minor manufactures, notably for the production of nitrogenous manure from the air.

Fig. 6.-A diagrammatic cross-section of a recently glaciated valley. AB, the mountain slope which rose above the ancient glacier and has therefore retained the sharp, unrounded forms due to ordinary weathering. BC, the shelf or shoulder, formerly covered by the ice, and therefore strewn with glacial débris. It now usually forms a pasture or alp. The dotted line connecting CC shows the probable form of the pre-glacial valley; CD, the rocky wall of the existing U-shaped valley on whose floor the river now flows.

Associated with the hanging valleys of Alpine regions is the presence of a curious shelf, shoulder, or "bench," which frequently lies on the top of the cliff from which the lateral streams spring (see figs. 6 and 7). Any one who has done some walking in the Alps, must have noticed a peculiar and often trying feature of any walk which leads up the side of the valley. This is that the walk begins with a very steep ascent, where the road or track zig-zags to and fro. After this steep and trying climb the walker reaches a broad shelf (BC in figs. 6 and 7), where the slope is much less, and where the extent of relatively level ground gives room for the erection of a huge hotel, or perhaps only of a group of chalets. This shelf is covered with fine herbage, destined to be cropped by the cows of the community.

Fig. 7.-An actual cross-section of the Lauterbrunnen valley. The vertical and horizontal scales are the same. B marks the edge of the cliff wall over which the streams leap in cascades. A is the position of the stream at the bottom of the U-shaped valley. BC marks the position of the shelf, largely occupied by the pastures or alps. Above them are rocky, unsmoothed slopes.

If the traveller continue his walk he will find that above this pasture ground or alp the slopes are again steep up to the mountain summits. Possibly, however, his walk has been to see a famous waterfall from above, and he will find that the streams which flow with relative slowness over the comparatively gentle slopes of the alp or shelf, will at some point tumble over the region up which he climbed, probably in a series of leaps or cascades.

The U-shaped valley, the "hanging" tributaries, the shelf or shoulder running along the upper part of the cliff wall which bounds the main valley, all these are striking features of glaciated regions. We shall not here discuss the probable causes of this striking "break of slope," so different from the characteristically continuous slopes of an ordinary mature river valley. As has been indicated, it is here that active controversy rages. It is, however, important to note that the shoulder or bench of which we have spoken was almost certainly once covered by the ice, its gentle slope indicating the original valley floor, before over-deepening took place.

The reason why pasture now grows upon it is that it is covered with fine glacial débris, which makes fertile soil. The fertile soil, which is often irrigated by milky water from existing glaciers, combined with the effect of altitude upon the plants, produces rich pasturage, and makes cattle-rearing an important alpine industry.

The next interesting feature of glaciated regions is the occurrence of those curious mountain forms which have special names in nearly every recently-glaciated region. Those gigantic arm-chair-shaped notches, high up on the mountain sides, which the Welsh call cwms, the Scotch corries, the French cirques, and the Germans kare, are very widespread in the Highlands of Scotland, in the mountains of Wales, in the Tyrol, and in other parts of the Alps (though they are not common in the Central region), and in North America as well as elsewhere.

A cirque (fig. 8) is shaped something like an office arm-chair. The floor has only a gentle downward slope, and often lodges a lake; or in other cases it is marshy, showing that a lake was once present. The back and sides are steep and precipitous. In some instances, if several cirques occur near together, the side walls may be eroded through, so that a shelf is produced, as one might produce a bench by putting two chairs side by side, and cutting away the contiguous arms. Very often, as one may easily see in the Highlands of Scotland, a series of cirques occur, one above the other, so that a climber proceeding from the valley floor upwards has a succession of steep "pitches," to use a mountaineering term, alternating with easy if wet walks across the floors of the successive cirques.

Fig. 8.-Diagram showing two glacial cirques.

It quite often happens in the case of high mountains in the Alps that the topmost of such a series of cirques still retains a glacier, what is called a dead glacier, that is, one which has practically ceased to move.

In other cases, again, we may find that what should be the flat floor of the cirque has been largely eaten away, as it were, by a huge rounded trough, which occupies what would be the extreme front of the seat of the arm-chair. In this trough a stream runs, and the trough has the characteristic U-shaped rounding characteristic of glacial forms. Further, at the top of the wall of the trough a bench or shelf exists, which is obviously the remains of the old cirque floor. In the case of all characteristic glacial cirques, however, the special feature is that the flat bottom of the cirque is discontinuous with the valley below; they are not parts of the same system of drainage. What we may call an unconformity appears between the two regions, more or less marked according as running water has or has not had time to begin the work of the removal of the unconformity.

The immediate human importance of these corries or cirques is not so apparent as in the case of hanging valleys, but they must be mentioned, if only because of their extraordinary abundance in glaciated regions, and especially in Great Britain. There are two views as to their origin, and we shall indicate both here without making any attempt to decide which is the correct one. A very full and clear statement of one position will be found in an article by Prof. Garwood in the Geographical Journal for September 1910, while previous articles by Prof. Davis and others in this journal formulate the opposed view.

To the first school the corrie is simply in origin the collecting basin of a pre-glacial stream, such a basin tending to acquire, roughly speaking, a flattish bottom and somewhat steep sides. With the onset of the ice the floor of the basin was protected by the ice from further erosion, while the frost ate back the wall and so steepened it, and the glacier carried away all débris as it formed. At a later stage the lower part of the glacier disappeared and only the cirque glacier was left. It continued its protective action, while below the powerful torrents hollowed out a trough. This process was perhaps repeated several times, with the final result that the protected cirque was left as a much-modified remnant of pre-glacial conditions, while the valley below was powerfully eroded by the glacial torrents. Thus a cirque lying above an existing valley is to be regarded as the beheaded end of an old valley, preserved by its ice covering, while below the old valley has been fundamentally modified by the scour of the glacial torrents. On this view the sharp distinction between the two angles of slope marks the distinction between the work of ice (protective) and the work of water (erosive). A series of cirques means a succession of glacial and interglacial periods.

According to the other school, for whom ice is a more powerful eroding agent than water, the cirque was produced by the ice, its presence or absence, in e. g. the Alps, being determined by the shape of the pre-glacial mountains. Cirques are believed to have been produced by the ice wherever the form of the mountains conduced to the accumulation of snow, and the occurrence of a series of cirques, and of the troughs which seem sometimes to eat into their floors, is ascribed to the successive retreat of the great ice-plough, i. e. to the action of the retreating ice itself, and not to the water which flows from beneath it.

* * *

Another striking feature of many glacial valleys is a very marked want of continuity in the slope of the main valley. Not only do the side valleys "hang" over the main valleys, but, further, this main valley itself often consists of relatively level reaches alternating with rocky bars, through which the river has sometimes later cut a gorge. Examples of this are very frequent. The famous gorge of the Aar above Meiringen is a river gorge cut through a rocky bar of this kind.

The Pyrenees are somewhat less familiar, both to tourists and in the form of pictures, but there, also, the same thing occurs. Above the health resort of Cauterets lies the little Lac de Gaube, whose mouth is blocked by a rocky bar through which the little torrent is cutting a tiny gorge. If the tourist crosses the lake in a boat and begins to walk up the valley above it, he will find that it has the form of a staircase, the huge steps being separated from one another by broad plateaux, which are flat and swampy, and have obviously been occupied by lakes not long ago. Above each plateau there is a rocky wall, almost precipitous, down which the stream flows in cascades. In other parts of the Pyrenees the same phenomenon occurs, and the lakes sometimes persist, lying one above the other in a series.

Fig. 9.-Profile of the Maderaner thal in Switzerland, to show the staircase arrangement peculiar to recently glaciated valleys. (From Garwood.)

The phenomenon is so common that it markedly affects human life in the Alps. The "landings," as the French call them, usually afford good pasture ground, while those which lie at no great elevation can be cultivated. Further, as the ground is level there is room for houses or even for a considerable village. The intervening region or step is too rocky to give level ground for human habitations or for pasture and cultivation. Where the river has had time to cut a gorge, the road must leave the stream, and can often be constructed only with difficulty. The result is that an Alpine valley often consists of a chain of villages, linked together by a difficult mule track or path. The abundant water-power, however, makes mechanical traction relatively easy, and we have sometimes the curious condition that a mule track is replaced by a railway, without the intervention of a road fit for wheeled traffic.

We need not stop to discuss the probable cause of this step and stair arrangement, which presents much the same problem as the series of cirques at the head of the valley. It is enough to indicate that according to one group of physical geographers the flat landings are due to the way in which the gradually decreasing glacier protected its bed from erosion, while the torrent which issued from it eroded very rapidly below; according to another school the landings are due to direct glacial erosion. There are other observers, again, who lay especial stress upon the modifications of the erosive powers of running water, due to the presence of the ice. For us it is of interest to notice that, as has been already indicated, the staircase effect occurs also, though on a smaller scale, in the case of mountain streams generally, some of which must be post-glacial in origin. In other words, there seems to be fundamental similarity between the work of ice and of water, the differences being differences of degree rather than of kind, and due largely to the varying fluidity of the two.

There is still one other feature of glaciated regions to which reference must be made. This is the occurrence of peculiarly open passes in considerable numbers across mountain regions which have been recently glaciated. In the geography books and in some maps, the Alps, for example, are represented as a great barrier, shutting off the fertile plains of Italy from the countries of Central Europe. But history shows that they have never been such a barrier, and the phrase of "splendid traitor" has been applied to the whole mountain range, in order to emphasise its total inadequacy as a barrier, either to armed or to peaceful invasion.

Since the time of Napoleon I public attention has been focussed upon a few great Alpine passes, notably the Mont Cenis, the Simplon and the St. Gothard, which are crossed by great carriage roads, now functionally replaced by railway tunnels beneath. But we must not forget that in addition to these and the other great passes there are almost innumerable ways of crossing the Alps on foot, and the presence either of Hospices or of small inns on many of the smaller passes shows that they are constantly used at the present time, in spite of railway tunnels and carriage roads elsewhere. Even a pass relatively so difficult as the Théodule, was used by very large numbers of Italian peasants during the time when work on the Simplon railway made great demands on Italian labour.

Any one of the passes, great or small, shows in outline the same characters. There is a steep ascent, often steeper on the Italian than on the other side, then a broad, windswept, open summit, sometimes almost level, below which the rapid descent begins. Not infrequently a lake, or lakes, may be found near the summit.

On a smaller scale the same phenomenon occurs in such glaciated regions as Scotland, the relatively low connections between one valley system and another greatly facilitating communication, and usually carrying both road and railway, where the latter exists. Such connections between two drainage systems (that is, the existence of a very low divide between the two) only exist on a small scale outside glaciated regions, so that they, with all their effects upon communications, must be largely ascribed to ice-action. We shall describe one case in a little detail, with the proviso that while no one denies the frequency of such passes in glaciated regions, some authorities believe that their production was due more to glacial torrents than to the erosive action of ice itself.

A very pretty example is the picturesque pass known as the Gemmi, which is traversed only by a mule path, and connects Kandersteg, and thus the lake of Thun and the town of Berne, with the Rhone valley, which the path enters at the village of Leuk. The walk proper is, however, over at the Baths of Leuk, a small health resort lying at the foot of the great Gemmiwand, a wall of rock over 1,600 feet in height on the summit of which is the Gemmi pass. Readers of Mark Twain's A Tramp Abroad will remember his interesting description of the crossing of the pass, which is part of the regulation tour in Switzerland.

The excursion may be very briefly described. The traveller starts from the village of Kandersteg, and almost immediately begins a steep climb, which after a rise of over 2,000 feet leads him over a ridge to a pasture, once swept by an avalanche. Another short but steep rise (note the staircase arrangement) leads him to the lonely Daubensee, a little lake which is frozen for more than half the year and has no outlet. It is itself fed by a glacier lying to the traveller's right, the Laemmern glacier, which is shrinking and exposing more and more of its old bed. Even to the most inexperienced traveller it is obvious that this present day shrinkage is, as it were, the last remnant of a shrinkage which has been going on for a prolonged period, so that the route by which the traveller ascended from Kandersteg is but a remnant of the bed of the old glacier. The point of special interest, however, is that at the end of the Daubensee the traveller leaves the glacial valley by which he has ascended, and passing through a great notch or gateway in a wall of rock, begins the almost precipitous descent to Leukerbad, which lies at his feet, 1,600 feet below. It is this notch which makes the pass, and it is fundamentally a breach in the mountain wall which separates the drainage of the Rhine from that of the Rhone. Comparing small things with great we may note that this gateway presents some resemblance to the Tyne and Aire Gaps in the Pennines, already mentioned, which may also have been modified by ice-action.

The explanation given is as follows:-At the time when the glaciation reached its maximum height the mass of ice in what is now the Laemmern glacier was so great that it could not be contained within its own valley. The ice was piled up so high that it over-rode the watershed, rose up beyond the containing wall of its own valley, and pushed a long arm over the valley wall, down into the Rhone valley. This tongue of ice, either by its own erosive power, or because of the glacial and sub-glacial streams which it produced, wore out a notch in the wall as it crossed, and it is this notch which makes the pass. As the glacier gradually shrank, it could no longer send this tributary over the wall into the valley below, and was constrained to send all its drainage into its own valley, that is ultimately into the Rhine. But the Gemmi pass persists as a proof of its former magnitude, of the fact that once part of the Laemmern drainage reached the Mediterranean instead of the North Sea, that there was once a communication between the Rhine and the Rhone drainage systems.

Many at least of the great Alpine passes are believed to have been produced in this way, and therefore we must add to the peculiarities of recently-glaciated countries, the fact that passes are likely to be frequent across their hills and valleys, owing to the power which ice possesses, when enormously developed, of rising above valley walls, and streaming down into another valley system. Some of the great Alpine passes, perhaps, arose in other ways, but this brief description may be of interest as suggesting one, probably common, mode of origin.

If we sum up what has been said as to the special features of glaciated regions, we may note that their valleys tend to be U-shaped, and to be discontinuous with their tributary valleys, which "hang" over them. On the top of the cliff from which these tributary streams leap is a shelf, which is clearly a portion of the floor of the pre-glacial valley and is covered by glacial débris. At the heads of the valleys there are often cirques or plateaux, which again are markedly discordant, hanging high above the valley below. In the main valley itself there are similar discordances, giving rise to a staircase arrangement. Finally, different valley systems often communicate with each other by passes, natural highways which hang high above both valley systems alike.

Obviously, however, we might replace this detailed summary by the simple statement that whereas in a region subjected only to the action of running water, there is a marked tendency to continuity of slopes throughout, a tendency more and more marked the longer the water acts, in glaciated regions there is an equally obvious discordance, a discontinuity of slope, most marked where water has not had time to begin its smoothing action. As every glaciated valley which we can study in detail has been subjected to the action both of ice and of water, it is a simple deduction that the discontinuity is due to the differential action of the two. This is the point of geographical importance, and to the geographer it is of minor importance to know whether it is the passive resistance of the ice which has caused the discontinuity, or whether it is the water which has been unable to keep pace with the activity of the ice.

* * *

There is one other point which must be alluded to even in this very brief consideration of the effect of the ice age upon the physical geography of the glaciated regions. This is the fact that it greatly modified the numbers and distribution of plants and animals throughout the areas affected. Obviously the covering of ice must have rendered a large part of Europe uninhabitable both for man and for the vast majority of animals and plants. In Europe, therefore, as also in North America, there must have been a southward sweep of all living organisms, driven from their original habitat by the onset of the cold period. But the conditions in the two continents differed greatly.

In North America, especially in the east, there are no transverse chains of mountains, there is no southern sea until the Gulf of Mexico is reached in lat. 30°, and even here Florida almost touches the tropic, and Mexico extends far beyond it. In this continent, therefore, the plants and animals, though driven far to the south, still found room to live and multiply, and had no great obstacle to cross either in their southward journey, or when they strove to re-annex their old territory as the cold conditions passed away again.

It is a curious fact that the forest trees of eastern Asia and of eastern North America show a remarkable resemblance to one another, and both regions are very rich in species and in genera. It is believed that this rich North American flora is a remnant of pre-glacial conditions, and that its persistence is due to the ease with which the trees obtained an asylum to the south during the period when the climate was most severe.

In Europe, in spite of the fact that the winter climate is much milder than in corresponding latitudes in North America, the number of kinds of forest trees is much less, there is little resemblance to those of Asia and the eastern United States, and the trees have generally a less southern aspect. This is the more remarkable in that trees of southern facies introduced from China and Japan and from the United States thrive admirably in Europe, showing that there is no climatic obstacle to their presence there. To mention only a few examples, the Tree of Heaven (Ailanthus glandulosa), so very common, even as a wild tree in many parts of the continent of Europe, was introduced from China, while the beautiful Sophora japonica, so frequently planted in towns, comes, as its name indicates, from Japan, and the various species of those beautiful flowering trees known as Catalpa are either American or Asiatic. The western plane (Platanus occidentalis), another favourite town tree, comes from the United States, and other American trees which are found very abundantly in towns in the warmer parts of Europe are the black walnut and the honey locust (Gleditschia tricanthos). Perhaps more striking than any of these is the case of the so-called false acacia (Robinia pseudacacia), which is as common over a great part of the continent of Europe as hawthorn bushes or wild roses are with us, and yet is a North American species, introduced less than three hundred years ago. Generally, we may say that all the more beautiful trees now growing in the warmer parts of Europe come either from eastern Asia or from the United States. In other words, the Ice Age seems to have greatly impoverished the flora of Europe. To a less extent this is also true of western North America, which has fewer species of trees than the east.

Why had the ice this impoverishing effect upon Europe? The topography of the continent supplies the answer. In the first place, in Europe there are numerous transverse chains of mountains. The Pyrenees, the Alps, the Caucasus, each with its load of ice, each with glaciers deploying on the low ground at its feet, must have been obstacles in the way of the southern migration alike of plants and of animals. Again, even if these obstacles were passed or turned, the great inland sea formed another barrier further south. In consequence of this difficulty in finding asylums the pre-glacial plants and animals must have perished in considerable numbers, and thus a general impoverishment took place. One must not of course exaggerate. A proportion of the pre-glacial forms did succeed in living through the period of stress, but many must have been, as it were, squeezed out of Europe or out of existence by the unfavourable climatic conditions.

As the climate improved the lands swept bare once again became inhabitable, and there was a recolonisation by movements from the south and from the east. We shall indicate later how man himself came from the south and the east to colonise the west and north, but his movements were only part of a great series which included also those of plants and animals.

* * *