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From the relations of the basaltic sheets of Antrim to the Upper Chalk, it is clear that the latter formation, after its deposition beneath the waters of the Cretaceous seas, was elevated into dry land and exposed to a long period of subaërial erosion before the first sheets of lava invaded the surface of the ground.
As regards this important point of the subaërial region of coal, I am glad to find myself in entire accordance with Principal Dawson, who bases his conclusions upon other, but no less forcible, considerations. In a passage, which is the continuation of that already cited, he writes:
How may a plain of marine abrasion be expected to differ from a peneplain in its mantle of waste? Compared with subaerial denudation, marine abrasion is a comparatively feeble agent.
On a rainy day, even in a gently undulating country, we see the effects of subaerial degradation in the muddy rills which flow down every slope. Messrs.
Although subaerial and submarine denudation, as before stated, remove, in the course of ages, large portions of the upper or more superficial products of volcanoes, yet these are sometimes preserved by subsidence, becoming covered by the sea or by superimposed marine deposits.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at great Depths. Why the Height of the successive Strata in a given Region is so disproportionate to their Thickness. Computation of the average annual Amount of subaerial Denudation. Antagonism of Volcanic Force to the Levelling Power of running Water.
They are, in short, loamy or clay soils, and must have been sufficiently above water to admit of drainage. The absence of sulphurets, and the occurrence of carbonate of iron in connection with them, prove that, when they existed as soils, rain-water, and not sea-water, percolated them. The coal and the fossil forests present many evidences of subaërial conditions.
The Grand Curral is the central vent of a volcano originally submarine, and, like the Peak of Tenerife, of the age miocene. Fossils of that epoch have been found upon the crater-walls of both. Subsequent movements capped it with subaerial lavas and conglomerates; and wind and weather, causing constant degradation, deepened the bowl and almost obliterated signs of igneous action.
But the continuous deposition of strata is often interrupted by movements of the earth's crust, Old sea floors are lifted to form land and are again depressed beneath the sea to receive a cover of sediments only after an interval during which they were carved by subaerial erosion.
Suppose, therefore, we had ascertained that the triturating power of subaerial denudation might in a given time in three, or six, or a greater number of millions of years pulverise a volume of rock equal in dimensions to all the present land, we might yet find, could we revisit the earth at the end of such a period, that the continents occupied very much the same position which they held before; we should find the rivers employed in carrying down to the sea the very same mud, sand, and pebbles with which they had been charged in our own time, the superficial alluvial matter as well as a great thickness of sedimentary strata would inclose shells, all or a great part of which we should recognise as specifically identical with those already known to us as living.
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