AN ESTIMATE OP THE GEOLOGICAL AGE OF THE EARTH. 279 



The conditions of subaerial denudation of the present suggest con- 

 siderable latitude within which the ratio of land to water may vary 

 without affecting the denudation 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. Murra}- to amount to 

 12.2 X 10" square miles.^ Over these regions the rainfall is less than 10 

 inches, and is reevaporated without reaching the sea. If the land area 

 were diminished by this number of square miles, the effect on the sup- 

 ply 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 

 probably increase; but the denudation to the ocean would probably 

 again be only aff'ected in a comparatively small degree. In the extreme 

 case of the entire land plateau being occupied by dry land, the disturb- 

 ance of balance might so far affect the amount evaporated from the 

 oceans as to diminish the land denudation. 



Many causes act to inffuence the rainfall on the earth. The larger 

 ones, as we have seen, will hardly act to produce great variations. 

 The smaller we can not 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 

 io just such rainless regions in the past as now exist. The most cau- 

 tious 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 sul)aerial denudation.^ 



Chemical denudation. — Quite another factor in the uniformity of 

 solvent denudation is the chemical and physical nature of the rock 

 surfaces and soils exposed during the successive ages of the earth's 

 historj^ With reference to the view that in earlier times larger areas 

 of igneous rocks were exposed to denudation than in more modern 

 periods, some remarks on soil and weathering are necessary. 



We see in soil formation of the present day a process of ever deep- 

 ening disintegration of the parent rock, and simultaneously progress- 

 ing decomposition of the upper layers. This resvdts in a surface layer, 

 possessing a reduced percentage of the more soluble materials, which 

 protects the richer material beneath. If the rock itself is for physical 

 or chemical reasons highly resisting, the leaching out of soluble 



^ Assuming that over areas with less than 20 indies rainfall there is complete 

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



"^ [Note added in the j^reits.] The possibilities of sun history, however, enter the 

 question. Professor Perry (Nature, July 13, 1899, p. 247) states it as his belief (in 

 reference to Professor Newcomb's view that sun heat can have varied but little during 

 Paleozoic 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 holil that in the past there 

 was much increased denudative activity should bear the possibility referred to by 

 Professor Perry in mind. 



