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surface has been brought under the plough, and has become arable land.

A channel however remains, through which the tidal waters pass and repass to the lake; and through which also the flood waters of the latter escape, when the surface of the lagoon has been raised by the rivers and streams that discharge into it during the rainy season. At such times its waters are almost or quite fresh; but during the greater part of the year, more or less sea water is intermingled, especially in the neighbourhood of the outlet channel, rendering them saline or brackish. Year by year, these lagoons become shallower, since most of the sediment brought in by the streams from the land is deposited on the bottom of the lake; and in the course of time, they will be filled up and will be converted into dry grassy plains. Some miles to the north of Pondicherry, near the village of Mercánum, there is such a lagoon, now nearly filled, and a great part of the low plain that extends at intervals along the Madras coast has been formed in this way and afterwards elevated.

In the last three chapters, we have reviewed briefly the work done on the surface of the earth by the waters condensed from the vapour of the atmosphere. Glaciers and rivers wear down the mountains, and plough out those valleys through which the waters find their way downwards to the sea; frost splinters and cracks the rocks; and rain water, dissolving the carbonic acid of the atmosphere, penetrates them, and works chemical changes in their mass, by which they are decomposed; some part of their constituents being dissolved by the absorbed water, while the residue, having lost its rigidity and hardness, yields easily to gravitation and the friction of running water. The materials, derived from the waste of the mountains, are spread abroad over the low plains at their foot, or carried further out to form river deltas, or finally to settle down on the sea bottom.

In most countries then, in all where the atmosphere is moist and the precipitation as rain or snow is abundant,

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THE WORK OF HEAT AND GRAVITY. [CHAP. IX.

the form of the surface is largely modified by these agents. And even in those arid regions where no rain falls, the deserts of Central Asia, the Sahára and the coast of the Red Sea, the deeply-scored sides of the mountains and tortuous wádis bear testimony to their former activity. These countries were not always so arid and rainless as they are now. In times geologically recènt, both the Sahára and a great part of Central Asia were occupied by the sea; the remnants of which, (in the latter case,) we have in the existing great salt lakes the Caspian and the Sea of Aral. When these seas existed, their evaporation furnished rain and snow, which falling on the slopes of the Atlas, the Altai and the Thian Shan, shaped and fashioned their peaks and ridges, much as the Himálaya is now being shaped and fashioned by the waters condensed from the evaporation of existing Indian seas.

Carrying our thoughts onwards from these agents to the causes which produce them and give them power, we find that the ulterior agents, whose work is thus unceasingly to wear down the land surface to one uniform level, are no other than those whose work they thus destroy. Heat and gravitation break up the earth's surface, causing continents to protrude from the ocean, and thrusting up broken and contorted layers of rock to form their mountain skeletons; and heat raises the ocean waters as vapour, which, condensed on the cold mountain sides, gravitate again to the sea, and wear away the surface in their passage. But the heat that upheaves mountains is the primeval heat of the earth; that which planes them down is the heat emitted by the sun.

CHAPTER X.

THE LAND AND ITS TENANTS.

In the preceding pages we have traversed, rapidly indeed, but it is to be hoped not inattentively nor unprofitably, some few galleries of the great workshop of nature. By the light of the facts furnished by the physical sciences, I have sketched out and briefly explained some of the more important processes by which the fair surface of our earth is incessantly wrought and fashioned. We have learned that not only things that live and die, but every part of our world is in a state of perpetual change, so that to-morrow's sun will rise on an earth changed in some of its aspects from that on which he sets to-day. Some of these indeed are so fleeting, like that of the bright meteor flashing across the midnight sky, that we must keep a watchful eye to seize them as they pass. Others, again, change so slowly, that in the brief span of a human life they seem absolutely fixed and stable; but could we, like the fabled Tithonus of old, live on through centuries, with senses and mind unimpaired, we should have to acknowledge that all, without exception, are but passing phases in the ever-changing appearance of our earth; that even the "everlasting hills" are falsely so called, and, by slow degrees, bow their lofty heads to the universal law of destruction.

The varied forms of the outline and surface of the land, as it now exists and as laid down on maps and charts, the variations of its climate, its fertility or barrenness, its

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132 PHYSICAL GEOGRAPHY PAST AND PRESENT. [CHAP.

populous or desert character, are then matters that we might completely explain, could we know the whole history of the changes that it has undergone from the earliest times; could we construct, for instance, maps of the world as it existed at successive and not too distant intervals of the geological past; and could we know for each of these epochs what climates prevailed, how the ocean currents then ran, how vapour-bearing winds then blew, in what measure snow and rain were then wasting the several parts of its surface, how the internal heat then manifested its changes in volcanic eruptions or in the slow uprising or depression of certain regions; and, far more completely than we now know them, the strange forms of trees and plants that clothed its surface, and of the animals that basked in their shade, or sped through the ocean waters of the younger world.

It need hardly be said that we are very far indeed from possessing this knowledge. The most ambitious attempts of geologists, in this direction, have as yet arrived at little more than to sketch out roughly some small parts of our continents, as they existed in the latest geological times: and, even in these times and places, the strange succession of almost tropical heat and arctic cold, to which the rocks and their fossils bear witness, where temperate climates now prevail, offer problems which science has not yet succeeded in explaining in a satisfactory manner.

Still, we may profitably take stock both of our knowledge and our ignorance in these matters; and while we note some of the more striking features of the distribution of land and water on the surface of our earth as we now see it, we may ask ourselves how much of these we can explain by the help of such facts as are recorded in the foregoing chapters, and how much still remains for future investigation and discovery.

First we must observe that by far the greater part of the land of our globe is situated in the northern hemisphere. In the southern hemisphere, besides the so-called antarctic continent, a mass of land around the south pole which is all but concealed beneath a perpetual sheet of ice, the

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only great tracts of land are portions of Africa and South America, and the insular mass of Australia, which may fairly claim to rank as a continent. Of this peculiar distribution of the land, science has as yet been unable to offer any explanation. That the earth beneath the ocean bottom in the southern hemisphere must be somewhat more dense than that forming the northern hemisphere, in order to compensate for its smaller volume, may be safely concluded from elementary mechanical considerations; but why this is so, or rather, how it has become so, we cannot say. Mr. Dana, following Scrope, Herschel, and Babbage, (albeit by a somewhat different road of deductive argument,) concludes that the general position of the great continental masses of land was determined at a very early period of the earth's history; so that, whatever changes of form the land has since undergone, two principal masses of land, corresponding to the old and new continents (so-called) have always been characteristic features of our globe. And Professor Ramsay, in a lecture before the Royal Institution, has lately pointed out that some of the ancient formations of Western Europethe old red sandstone for instance, and the Triassic marls, which abound in salt-are the deposits of ancient lakes, and indicate that in these remote times there must have been an extensive tract of land surrounding the areas they Occupy.

But on the other hand, it is certain that the greater part of the present land, at one time or another, has been covered by the sea, and for such long periods that deposits many thousands of feet in thickness have had time to accumulate; so that, even if we admit the justice of Professor Dana's conclusions, it is clear that very great changes have taken place in the forms of the continents, while it is equally certain that considerable areas of the present oceans have at various times been occupied by tracts of land. The great plains of Northern Asia and Europe have been upraised from the sea in very recent geological times; at a somewhat earlier but still comparatively recent period, (the Miocene,) a sea occupied the central part of Europe and