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



although the weathering as dependent on chemical decomposition is most active 

 on the eruptives.* The effect is greatly physical in origin. " In stratified rocks 

 there is, as a rule, a lack of homogeneity, certain layers being more porous 

 than others, or containing mineral constituents more suscej^tible to the attacking 

 forces."! 



A full account of the conditions at work, so far as our present knowledge 

 extends, appears in Merrill's work, already so frequently referred to. 



The entire consideration shows that the greater richness in alkalies of the 

 original igneous rocks is conjoined to such resistant physical properties as in the 

 general case involves the more rapid turn over of the less rich sediments. The 

 frequently greater richness of the residual soils of the latter is a consequence of 

 this. 



But apart from such considerations, have we any valid reason to expect in* the 

 past a more rapid solution of the rocks than progresses at the present day ? Factors 

 enter the question on each side. The denser atmosphere of carbonic anhydride 

 which may have obtained in the Palaeozoic epoch, and whicli would have contribu- 

 ted not only more carbonic acid to the rain, but by its great pressure have enabled 

 this to take up a greater quantity, finds a set-off iia the subsequent much 

 greater development of vegetation. The humic and allied acids exert, as is now 

 known, a powerful influence in promoting decomposition. " There is reason to 

 believe that, in the decomposition effectedby meteoric waters, and usually attributed 

 mainly to carbonic acid, the initial stages of the attack are due to the powerful 

 solvent capacities of the humus acids."J 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 Palaeozoic epochs. Indeed, if the dense atmo- 

 sphere of carbon dioxide existed, its mere mechanical effects when urged to the 

 speed of a gale would have sufficed to destroy any but lowly plants in sheltered 

 positions. § 



Tlie carbonic anliydride of the atmosphere of to-day by no means corresponds 

 in amount with that which effects the operations going on in the soils. The per- 

 centage of CO, in soils is far greater than in tlie air. The decay of vegetation is 

 probably ultimately responsible for this increase. While the CO2 in 10,000 parts 

 by weight of the atmosphere may be about six, that in soils, rich ir humus, may 



* Merrill, he. cit., p. 271. The familiar appearapce of igneous dykes standing out like oralis above 

 surrounding sedimentaiy rocks is an example. 



t Loc. cit., p. 248. \ Geikie, " Text Book of Geology," 3rd edition, p. 472. 



§ Possibly tbese mechanical effects may be accountable for the earlier forest vegetation 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. 



