Diagram of proportionate mean height of Land and depth of Oceans.
Land
Area. .28 of area of Globe. Ocean
Area .72 of area of Globe.
In this diagram the lengths of the sections representing land and ocean are proportionate to their areas, while the thickness of each is proportionate to their mean height and mean depth respectively. Hence the two sections are in correct proportion to their cubic contents.
A mere inspection of this diagram is sufficient to disprove the old idea, still held by a few geologists and by many biologists, that oceans and continents have repeatedly changed places during geological times, or that the great oceans have again and again been bridged over to facilitate the distribution of beetles or birds, reptiles or mammals. We must remember that although the diagram shows the continents and oceans as a whole, yet it also shows, with quite sufficient accuracy, the proportions of each of the great continents to the oceans which are adjacent to them. It must also be borne in mind that there can be no elevation on a large scale without a corresponding subsidence elsewhere; because if there were not a vast unsupported hollow would be left beneath the rising land or in some part adjacent to it.
Now, looking at the diagram and at a chart or globe, try to imagine the ocean-bottom rising gradually, to form a continent joining Africa with South America or with Australia (both of which are demanded by many biologists): it is clear that, while such an elevation was going on, either some continental land or some other part of the ocean-bed must sink to a corresponding amount. We shall then see, that if such changes of elevation on a continental scale have taken place again and again at different periods, it would have been almost impossible, on every occasion, to avoid a whole continent being submerged (or even all the continents) in order to equalise subsidence with elevation while new continents were being raised up from the abyssal depths of the ocean. We conclude, therefore, that with the exception of a comparatively narrow belt around the continents, which may be roughly indicated by the thousand fathom line of soundings, the great ocean depths are permanent features of the earth's surface. It is this stability of the general distribution of land and water that has secured the continuity of life upon the earth. Had the great oceanic basins, on the other hand, been unstable, changing places with the land at various periods of geological time, they would, almost certainly, again and again have swallowed up the land in their vast abysses, and have thus destroyed all the organic life of the world.
There are many confirmatory proofs of this view (which is now widely accepted by geologists and physicists), and a few of them may be briefly stated.
1. None of the continents present us with marine deposits of any one geological age and occupying a large part of the surface of each, as must have been the case had they ever been sunk deep beneath the ocean and again elevated; neither do any of them contain extensive formations corresponding to the deep oceanic clays and oozes, which again they must have done had they been at any time raised up from the ocean depths.
2. All the continents present an almost complete and continuous series of rocks of all geological ages, and in each of the great geological periods there are found fresh water and estuarine deposits, and even old land-surfaces, demonstrating continuity of continental or insular conditions.
3. All the great oceans possess, scattered over them, a few or many islands termed 'oceanic,' and characterised by a volcanic or coralline structure, with no ancient stratified rocks in anyone of them; and in none of these is there found a single indigenous land mammal or amphibian. It is incredible that, if these oceans had ever contained extensive continents, and if these oceanic islands are—as even now they are often alleged to be—parts of these now submerged continents, not one fragment of any of the old stratified rocks, which characterise all existing continents, should remain to show their origin. In the Atlantic we find the Azores, Madeira, and St. Helena; in the Indian Ocean, Mauritius, Bourbon, and Kerguelen Island; in the Pacific, the Fiji, Samoan, Society, Sandwich, and Galapagos Islands, all without exception telling us the same tale, that they have been built up from the ocean depths by submarine volcanoes and coralline growths, but have never formed part of continental areas.