116 
ELEMENTS OE GEOLOGY. 
continually to reduce the whole of the land to the level of 
the sea, the other to restore and maintain the inequalities of 
the crust on which the very existence of islands and conti¬ 
nents depends. I stated, however, that when we endeavor 
to form some idea of the relation of these destroying and 
•renovating forces, we must always bear in mind that it is 
not simply by upheaval that subterranean movements can 
counteract the levelling force of running water. For where¬ 
as the transportation of sediment from the land to the ocean 
would raise the general sea-level, the subsidence of the sea- 
bottom, by increasing its capacity, would check this rise and 
prevent the submergence of the land. I have, indeed, en¬ 
deavored to show that unless we assume that there is, on 
the whole, more subsidence than upheaval, we must suppose 
the diameter of the planet to be always increasing, by that 
quantity of volcanic matter which is annually poured out in 
the shape of lava or ashes, whether on the land or in the bed 
of the sea, and which is derived from the interior of the earth. 
The abstraction of this matter causes, no doubt, subterranean 
vacuities and a corresponding giving way of the surface; if 
it were not so, the average density of parts of the interior 
would be always lessening and the size of the planet increas-- 
ing.* 
Our inability to estimate the amount or direction of the 
movements due to volcanic power by no means renders its 
efficacy as a land-preserving force in past times a mere mat¬ 
ter of conjecture. The student will see in Chapter XXIV. 
that we have proofs of Carboniferous forests hundreds of 
miles in extent which grew on the lowlands or deltas near 
the sea, and which subsided and gave place to other forests, 
until in some regions fluviatile and shallow-water strata with 
occasional seams of coal were piled one over the other, till 
they attained a thickness of many thousand feet. Such ac¬ 
cumulations, observed in Great Britain and America on op¬ 
posite sides of the Atlantic, imply the long-continued exist¬ 
ence of land vegetation, and of rivers draining a former con¬ 
tinent placed where there is now deep sea. 
It will be also seen in Chapter XXV. that we have evi¬ 
dence of a rich terrestrial flora, the Devonian, even more an¬ 
cient than the Carboniferous; while on the other hand, the 
later Triassic, Oolitic, Cretaceous, and successive Tertiary 
periods have all supplied us with fossil plants, insects, or 
terrestrial mammalia; showing that, in spite of great oscilla¬ 
tions of level and continued changes in the position, of land 
and sea, the volcanic forces have maintained a due propor- 
* Principles, vol. ii., p. 237 ; also 1st ed., p. 447. 1830. 
