54 Joty—An Estimate of the Geological Age of the Earth. 
VII.—Uniformity of Denudation by Solution. 
Land area and Rainfall_—The most prominent considerations involved in the 
question of how far the present rate of denudation by solution may be accepted 
as an average of that extending over past times are that of the varying ratios of 
land and sea areas of the past and the amount of rainfall received upon the latter. 
The fact that paleeontologically similar deposits in the various parts of the 
world are not necessarily contemporaneous, but homotaxial, debars the geologist 
from mapping the sediments of any horizon (even were these fully known) as 
forming part simultaneously of the oceanic area. Could he even claim full 
assurance here, the land areas supplying the sediments must still remain unknown. 
In this difficulty indirect inferences only can be resorted to. 
Those who accept the stability of the continents and oceans as a whole cannot well 
admit that the balance of land and water was ever very seriously interfered with. 
Sir J. Murray* has calculated that if the present land of the globe were reduced 
to the sea level by being removed to and piled up in the shallow waters of 
the ocean, its extent would be altered from the present 55 x 10° to 80 x 10° 
square miles, the ocean simultaneously changing from 137-2 x 10° to 113 x 10° 
square miles. ‘The mean height of the land, which is at present 2250 feet, would 
become 0; while the mean depth of the ocean, at present 2080 fathoms, would 
increase to 3 miles, 23°45 x 10° cubic miles of material being transported into the 
sea. 
If the Earth’s crust were rigid, and neither subsidence or elevation ever took 
place, such a calculation would mark the extreme distribution of the existing 
sub-aerial material which would be possible under the action of denuding agencies. 
It could only be brought about by an infinitely prolonged denudation and 
quiescence of the crust. 
As a matter of fact, however, we know that over the continental areas there 
have been frequent depressions and elevations, and these acting alternately again 
and again over the same area. The Uniformitarian, we assume, regards this 
shifting balance of land and water as confined mainly to the area indicated above, 
the 80 million square miles marking out the elevated plateaux of the globe. The 
dry land of to-day occupies some 68 per cent. of this area. It cannot be 
supposed to have ever occupied 100 per cent. of it, for then sediments must have 
been laid down in the present ocean troughs. That such sedimentation, again, as 
the glass, and the taking up of a small additional amount of potash and soda (apparently from the sea)—is 
hardly sufficiently abundant, according to present knowledge, to justify consideration here. See the Report 
on the Deposits, p. 304. The Phillipsite appears to be a purely alteration product of the basic débris. See 
Merrill (Joc. ctt., p. 375). 
* Scottish Geological Magazine, 1888, pp. 1 et seq. 
