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Evidence of the amount of denudation which the land has undergone Sub-aerial denudation The deposition of dust Vegetable mould, its dark colour and fine texture largely due to the action of worms The disintegration of rocks by the humus-acids Similar acids apparently generated within the bodies of worms The action of these acids facilitated by the continued movement of the particles of earth A thick bed of mould checks the disintegration of the underlying soil and rocks.
We are now prepared to consider the more direct part which worms take in the denudation of the land. When reflecting on sub-aerial denudation, it formerly appeared to me, as it has to others, that a nearly level or very gently inclined surface, covered with turf, could suffer no loss during even a long lapse of time.
A. Tylor had adduced important evidence on sub-aerial denudation, by showing that the amount of matter brought down by rivers must infallibly lower the level of their drainage basins by many feet in no immense lapse of time. This line of argument has since been followed up in the most interesting manner by Archibald Geikie, Croll and others, in a series of valuable memoirs.
The enormous pressure of the water on their sides enables these mid-oceanic islands to stand with slopes varying from the perpendicular to a smaller extent than if they were sub-aerial; and it is on this account that we find them rising with such extraordinary abruptness from the "vasty deep." The volcanic islands of this great ocean are scattered over a wide tract on both sides of the equator.
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