282 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



carbonic acid to the rain, but by its g-reat pressure have enal)led this 

 to take up a g-reater quantity, tinds a set-ofl" in the 8u})sequent nuich 

 greater development of vegetation. The huniic and allied acids exert, 

 as is now known, a powerful influence in promoting decomposition. 



"There is reason to believe that, in the decomposition eftected by 

 meteoric waters, and usually attributed mainl}- to carbonic acid, the 

 initial stages of the attack are due to the powerful solvent capacities 

 of the humus acids.'" ^ 



The mechanical action of the roots is also a very important factor. 

 Now, these effects of vegetation were probably absent during the Pre-. 

 Cambrian and early Paleozoic epochs. Indeed, if the dense atmos- 

 phere of car})on dioxide existed, its mere mechanical efl'ects when 

 urged to the speed of a gale would have sufficed to destroy any but 

 lowly plants in sheltered positions.^ 



The car])onic anhydride of the atmosphere of to-day by no means 

 corresponds in amount Avith that Avhich effects the operations going on 

 in the soils. The percentage of CO.^ in soils is far greater than in the 

 air. The decay of vegetation is probably ultimately responsible for 

 this increase. While the COo in 1(),()00 parts ])y weight of the atmos- 

 phere may be about 6, that in soils rich in hunuis ma}^ rise to 548 

 parts. ^ This is the atmosphere actually concerned with the destruc- 

 tion of feldspars, etc. 



The existing soils of a considerable part of the Northern Hemisphere 

 are due to the glacial effects of older Quaternary times. However, in 

 the loess of China, Europe, etc., the adobes of America, and similar 

 clavs, surface deposits are found which ma\' well have been represented 

 in the remote past. In these we find alkali percentages comparable 

 with the sedimentary soils, the potash ranging from 1.03 to 2.13, the 

 soda from 0.57 to 1.63. The state of conuninution is also remarkable.* 



The interesting evidence of pre-Paleozoic granitic decay described 

 by Dr. R. Bell, of the Canadian Geological Survey, and referred to by 

 Merrill,'^ should be referred to by those interested in the question, 

 although, as not being of a quantitative nature, the evidence does not, 

 save for its general teaching, concern us here. Other cases of evidence 

 for pre-Cambrian denudation are mentioned in the same treatise. Mr. 

 Merrill concludes: 



"These and other illustrations that might be given yjoint unmistak- 

 ably to the identity of geological processes and correspondence in 

 results since the earliest times, even did not analogy and the thousands 

 of feet of secondary rocks furnish us safe criteria "upon which to base 

 our inferences. " 



^Geike, Text Book of Geology, third edition, p. 472. 



^Possibly these mechanical effects may be accountable for the earlier forest vege- 

 tation possessing the morphological characters of that now clothing exposed and 

 mountainous regions rather than those of the leafy trees of our valleys and plains. 

 Its habitat, moreover, appears to have been the marsh and the sunken place. 



^'Merrill, loc. cit., p. 178. 



*Loe. cit., p. 330. 



*Loc. cit., pp. 275, 276. 



