ox THE GEOLOGICAL AGE OF THE EARTH. 371 



How far will this measure of uniformity be conceded ? Whether 

 crystalline or sedimentary rocks jwevail does not appear seriously involved 

 in the long run, for we find soils derived from the latter actually exposing 

 larger amounts of alkaline silicates : the higher resistance to disintegra- 

 tion offered by the crystalline rocks often conferring upon their soils the 

 role of an exhausted and protective covering. Again, it is only within 

 fairly wide limits a question of climate, for the rate of solution of the 

 silicates is so slow that the amount of the solvent present is of less import- 

 ance than its persistent operation even in minute quantities. The rate of 

 solution would certainly not increase proportionately to the amount of the 

 solvent, a very wet climate being very possibly, even probably, less 

 effective than a warm and damp climate more rarely visited by rains. 



As regards the rainy area exposed to SHhai'rial denudation during past 

 geological periods, considerable latitude with respect to the effects of up- 

 heaval or depression is suggested by the fact that the supply of water 

 evaporated by the ocean is to-day insufficient to ensure drainage from 

 more than four-fifths the total land area. If to-day 10 per cent, of 

 the land subsided beneath the ocean, probably but a small change in the 

 river discharge of dissolved matter would I'esult. The disappearance of 

 this 10 per cent, of the land would increase the oceanic area but 4 per 

 cent., and the ' rainless ' areas of the continents would diminish to one- 

 tenth the total land surface. In short, the rainy margins would — 

 roughly speaking — move inwards. In the opposite case, that of upheaval, 

 the rainy margin will move outwards. Tlius the deposition or upheaval 

 of our greatest sedimentary masses was not necessarily accompanied by 

 any notable variation in the supply of dissolved matter to the sea. 



In short, it would appear that changes of a quite abnormal or cata- 

 strophic nature must be looked for to seriously affect the average rate of 

 the operations at Avork. I must refer to the evidence for and against .such 

 effects. 



As regards hydrothermal actions, due to lingering heat in the primitive 

 oceans, on Lord Kelvin's figures for the rate of cooling this action must be 

 negligible. If his figures were multiplied even tenfold, the error could 

 hardly at most amount to 1 per cent. I may also now refer to the 

 objection urged by Professor SoUas, that underground temperature may for 

 long have given rise to a geyser-like action of springs which would have 

 enriched the sodium supply of the early rivers. If of a serious character 

 this objection should be supported by more evidence of such solvent actions 

 than our most ancient sediments reveal. Thus it is remarkable that so far 

 from the earliest sediments or their probable metamorphosed remains 

 being the most washed-out of the rocks they are often those possessing the 

 largest percentages of alkalies. The oldest Cambrian and Silurian sedi- 

 ments and gneissic rocks of archsean age and probable sedimentary origin 

 show percentages of alkalies almost comparable to those of the mean 

 igneous earth-crust and exceeding the average of later sediments and of 

 sediments at present beiug deposited. I therefore cannot think that any 

 exceptional solvent actions applied to these sediments when being deposited 

 or when buried or subsequently when uplifted and exposed to atmo- 

 spheric denudation can be generally assumed. Gey.ser actions or circula- 

 tion of underground waters among unfaulted primitive igneous rocks, on 

 the other hand, would arise only under exceptional conditions. And, 

 again, we may ask, where in the earlier igneous rock-masses have we 

 evidence of exceptional geyser-like actions 1 



B B 3 



