118 
ELEMENTS OF GEOLOGY. 
tiaii rocks, and situated where the Atlantic now prevails. 
Such an hypothesis would be in perfect harmony with the 
conclusions forced upon us by the study of the present con¬ 
figuration of our continents, and the relation of their height 
to the depth of the oceanic basins; also to the considerable 
elevation and extent sometimes reached by drift containing 
shells of recent species, and still more by the fact ofi sedi¬ 
mentary strata, several thousand feet thick, as those of cen¬ 
tral Sicily, or such as flank the Alps and Apennines, contain¬ 
ing fossil mollusca sometimes almost wholly identical with 
species still living. 
I have remarked elsewhere^ that upward and downward 
movements of 1000 feet or more would turn much land into 
sea and sea into land in the continental areas and their bor¬ 
ders, whereas oscillations of equal magnitude would have no 
corresponding effect in the bed of the ocean generally, be¬ 
lieved as it is to have a mean depth of 15,000 feet, and which, 
whether this estimate be correct or not, is certainly of great 
profundity. Subaerial denudation would not of itself lessen 
the area of the land, but would tend to fill up with sediment 
seas of moderate depth adjoining the coast. -The coarser 
matter falls to the bottom near the shore in the first still 
water which it reaches, and whenever the sea-bottom on 
which this matter has been thrown is slightly elevated, it 
becomes land, and an upheaval of a thousand feet causes it 
to attain the mean elevation of continents in general. 
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—pul¬ 
verize a volume of rock equal in dimensions to all the pres¬ 
ent 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 recognize as 
specifically identical with those already known to us as liv¬ 
ing. Every geologist is aware that great as have been the 
geographical changes in the northern hemisphere since the 
commencement of the Glacial Period, there having been sub¬ 
mergence and re-emergence of land to the extent of 1000 feet 
vertically, and in the temperate latitudes great vicissitudes 
of climate, the marine mollusca have not changed, and the 
* Principles, vol. i., p. 265. 1867. 
