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Whether the four great depressions have always existed, causing the distribution of land and water to resemble that of to-day, is an interesting investigation, and geologists have not all come to the same conclusion. It is quite certain that many parts of the earth have been dry land at one time and sea at another; nearly all England, for example, has been many times submerged by the sea and uplifted again. What is not established is whether these interchanges of sea and land are confined to the neighbourhood of shallow seas and to the margins of the ocean basins. Some lands at any rate, among them Labrador, Scandinavia, some part of India, and much of Africa, have never been entirely covered by the sea during geological time. Yet the distribution of animals and plants leads us to think that the arrangement of land and water has differed greatly at different geological periods.
We can see as we examine a globe or a map of the world that this arrangement follows a definite plan. Even the earliest geographers were impressed with this, though their knowledge of the earth's surface was too limited for them to come very near the truth. The classical geographers imagined that the Mediterranean basin was the centre of a wheel-shaped island bounded by one vast ocean. The medieval geographers drew the lands of the globe radiating like spokes from Jerusalem as a centre. From a modern globe or map we shall see that the plan of the earth has four main features :
1. The Northern Hemisphere contains the greater part of the land and the Southern Hemisphere the greater part of the sea.
2. The lands form nearly a complete circle round the Northern Hemisphere and stretch southward in three pairs of continents : North and South America, Europe and Africa, Asia and Australasia. The oceans form a complete circle round the Southern Hemisphere and stretch northward.
3. Most of the lands and seas are irregular triangles.
The lands taper southward, and in consequence the seas taper northward.
4. Land and sea are antipodal—that is, a line drawn from a land surface to the centre of the earth and continued till it reaches the surface again is almost certain to have sea at the end of it. Thus the Arctic Ocean is antipodal, or opposite, to the Antarctic continent, Europe and Africa are antipodal to the Central Pacific, and so on.
The arrangement of land and water has a very close connexion with the habitability of the globe. Lands depend upon moisture for their fertility, and the oceans are the reservoirs from which their water supply is replenished by the agency of rain-bearing winds. Since the winds are drained of their moisture as they pass over the land, any region situated too far from the sea is desert. An alternation of land and sea will therefore render a much larger proportion of the land surface habitable than if all the land had formed one vast continent and the water one vast ocean.
If we speculate about the appearance of the infant earth, when its crust had solidified and cooled sufficiently to allow the water in the atmosphere to condense upon its surface, we may imagine a tumbled confusion of hills and valleys still warm and moist, warm, stagnant seas, not yet salt, not yet ruffled by the winds that we know to-day, and, wrapping it round, an atmosphere laden with carbon dioxide and thick with clouds, scarcely to be penetrated by the sun's rays. Under this cloud-screen the temperature would vary hardly at all, day or night, summer or winter. It would be a grey earth under such a sky, with no sunshine to bring out the colour of the distorted rocks. Not an atom of living green would be visible, for no life such as we know it could exist when the primeval crust was solidifying out of molten matter. It would take ages of preparation before the earth could be fit for the most primitive form of such life. Before it appeared the original rock surface would have to be broken down by destructive agencies existing from the beginning and working from the beginning, and rebuilt into new rock layers under whose pressure the solidification of the deeperseated rocks would proceed. As the contraction of the globe, more rapid in the interior, went on, fractures would occur in the crust and parts of it sink, and the
added pressure on the softer rocks beneath would drive them to escape through the vents, liquefying as the pressure was removed, in volcanic eruption. Once on the surface they would be exposed to the same destructive agencies as the older rock. Running water was already at work. Acid gases were present in the atmosphere; and when the sun could penetrate the screen of clouds it would start the winds blowing, the waters circulating, and give the earth its seasons. All that was necessary for the fashioning of the earth and the development of its life was already in existence. But the germ of life had yet to appear, and the preparation was long before the earth was fit to receive it. The preparation and the development as we see it to-day have taken perhaps a hundred million years.
THE MOVEMENTS AND FASHIONING OF
O life as we know it existed in the infant earth, but the materials necessary for life in every
form from the humblest to the highest were there, in the rocks, the water, or the air. Only as yet they were useless ; they were scattered here and there in minute quantities, or locked up at great depths, or in a form that would have to be broken up before its materials could be usefully employed. Much preparation of the crust was essential before the germ of life could thrive, and then life in its lowest form had to smooth the way for a higher stage.
It has been said that the original rocks of the crust had to be broken down and reformed, and that the agents which could carry out this work were in existence at the very beginning of the solid earth. It would be the higher surfaces of the rocks that would be most exposed to destructive agencies, and, once broken down, wind and water would transport them to lower levels and lay them down in horizontal sheets. By themselves these processes would in time result in the reducing of all land surfaces to sea-level ; they are counteracted by uplifts and sinkings in the crust itself and by balancing movements in adjacent areas.
As regards the latter, it would seem that the additional weight of a new layer of rock débris causes the area on which it is deposited to sink, while the neighbouring