AGE OF THE EARTH—JOLY. 275 
one-fifth of the whole. Under such circumstances transgression of 
the ocean upon the land simply results in the diminution or disap- 
pearance of the great continental desert regions. It has been shown 
by Murray that it would require a vertical depression relatively to the 
ocean of 600 feet in order to reduce the existing land area by 26.7 
per cent. Penck, on the same data, concludes that a submergence 
of 200 meters would reduce the area 29 per cent. A submergence 
of nearly 1,500 feet is required to diminish the land area 50 per cent. 
It is for geologists to judge whether world-wide transgressions of 
these magnitudes obtained for any long periods in the past. So far 
as I know, paleography would not support such transgressions. A 
recent study of the Paleography of North America by C. Schuchert 1 
leads to the conclusion that the mean area of that continent through- 
out the past has been about eight-tenths of its present area. In his 
Traité de Géologie, De Lapparent, in a series of well-known restora- 
tions of ancient geography, shows how far, as judged by the sedi- 
ments, there was transgression of the sea upon the land at various 
epochs. It does not appear that we can infer, even at the climax of 
the great Cenomanian transgression, that the existing land was at 
any time covered to one-half its extent. And mindful of the fact 
that the area of denudation is in most cases much greater than that 
of deposition, when the latter is greatest the necessity of accounting 
for the former involves the assumption that tracts of land now sub- 
merged were then exposed. Without assuming the former exist- 
ence of lost continents in the central oceanic basins, there seems 
very strong evidence for the disappearance of former land. The 
evidence is found in our own islands, in North America, in India, 
South Africa, and Australia and elsewhere. We have to recognize 
continual fluctuations, but the evidence for a prevailing reduction of 
continental areas by as much as 50 per cent, or even 25 per cent, in 
the past is, so far as I know, not forthcoming. We might go further 
and state that so great a diminution of existing land area as 50 per 
cent certainly did not prevail in the past. Such a reduction in- 
volves about 25 per cent of the present rate of solvent denudation 
and increases the age accordingly. 
Meteorological conditions, unless occasioned by a prevailing change 
in the amount of solar heat, can not be supposed to have steadily 
affected in one direction the rate of denudation. It is worthy of 
note that the testimony derived from the solvent denudation of the 
continents shows that climatal conditions do not, within the limits, 
seriously affect the rate of solvent denudation. This finds explana- 
tion in the extremely complex nature of the factors concerned in 
rock weathering and rock solution. Now, the mere abundance of 
1 Bull. Geol. Soc. America, vol. 20, 1910. 
