JoLY — An Estimate of the Geological Age of the Earth. 55 



we see in the great formations could have been effected without large areas 

 of exposed land is impossible. These rocks infallibly assert the existence of dry 

 land proportional to their own magnitude and complimentary to their own submer- 

 gence. The sedimentary deposits themselves suggest, then, from the necessities 

 of their supply, a limit on the other side, that is, to the reduction of land area in 

 past times.* 



The conditions of sub-aerial denudation of the present suggest considerable 

 latitude within which the ratio of land to water may vary without affecting the denu- 

 dation to the ocean. This is shown in the fact that the present amount of rainfall 

 on the land is not sufficient to denude more than four- fifths of its area into the 

 sea. The rainless regions of the Earth are estimated by Sir J. Murray to amount 

 to 12 -2 X 10'' square miles. f Over these regions the rainfall is less than 10 inches, 

 and is re- evaporated without reaching the sea. If the land area Avere diminished 

 by this number of square miles, the effect on the supply to the ocean would 

 probably be but small. If, on the other hand, it increased beyond its present 

 extent, the rainless area would also most pi-obably increase ; but the denudation 

 to the ocean would probably again be only effected in a comparatively small 

 degree. In the extreme case of the entire land plateau being occupied by dry 

 land, the disturbance of balance might so far effect the amount evaporated from 

 the oceans as to diminish the land denudation. 



Many causes act to influence the rainfall on the Earth. The larger ones, as 

 we have seen, will hardly act to produce great variations. The smaller we cainiot 

 suppose, reasonably, will always conspire to act one way. We have already 

 referred to the fact that, if the non-oceanic origin of the Rock Salt beds be 

 accepted, these deposits point to just such rainless regions in the past as now 

 exist. The most cautious conclusion, we submit, must be that the facts of Earth- 

 history over Geological Time, as we know them, do not point to any great or 

 long-continued changes in the conditions of sub-aerial denudation. J 



* See 'Wallace's " Island Life," chap. vi. See also Green's "Pliysical Geology," 1892, p. 687, ct seq., 

 and "Three Cruises of the Blake," by A. Agassiz, 1888, pp. 126 and 166. The question of the 

 permanence of continents and oceans has been so much discussed that further reference here is 

 unnecessary. 



f Assuming that orer areas with less than 20 inches rainfall there is complete re-evaporation, only 

 36,697,400 square miles actually drain into the sea. Loc. cit. 



\ \_Note added in the Press.'] The possibilities of Sun-history, however, enter the question. Prof. 

 Perry {Nature, July, 13, 1899, p. 247), states it as his belief (in reference to Prof. Newcomb's view 

 that sun-heat can have varied but little during Palaeozoic time), that there may have been millions of 

 years, during which the sun may have been radiating at only one-third or one-tenth, of its present rate. 

 This would of course lead to diminished meteorological activity generally, although the denudative 

 effects due to ice might increase. Those who hold that in the past there was much increased denudative 

 activity should bear the possibility referred to by Prof. Perry in mind. 



TKiNS. liOT. DUB. SOC, N.S. VOL. VII., TAKT III, ^ 



