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 dimmish 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 

 Traite" de Geologie, 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 



i Bull. GedI. Sbc. America vol. 20, lVlO. 



