58 Joty—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 susceptible 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. ‘he denser atmosphere of carbonic anhydride 
which may have obtained in the Palaeozoic epoch, and which 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 in the subsequent much 
greater development of vegetation. ‘The humic and allied acids exert, as 1s now 
known, a powerful influence in promoting decomposition. ‘‘ There is reason to 
believe that, in the decomposition effected by 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.” The mechanical action of the roots is also 
avery important factor. Now these effects of vegetation were probably absent 
during the Pre-Cambrian and early Paleozoic epochs. Indeed, ¢/ 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.§ 
The carbonic anhydride 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 the air, ‘The decay of vegetation is 
probably ultimately responsible for this increase. While the CO, in 10,000 parts 
by weight of the atmosphere may be about six, that in soils, rich iv humus, may _ 
* Merrill, Joc. crt., p. 271. The familiar appearance of igneous dykes standing out like walls above 
surrounding sedimentary rocks is an example. 
| Loe. cvt., p. 248. t Geikie, ‘“‘ Text Book of Geology,” 3rd edition, p. 472. 
§ Possibly these 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. 
