AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 281 



In the above it appears that the soils derived from the ig-neous 

 rocks — more especially the more basic ones — show a g-reater jxnerty 

 in alkalies than those derived from limestones and sandstone.;. This 

 probably arises, in part, from more soluble alkali constituents being 

 present, but in manj- cases doubtless from a more resistant parent 

 rock leading- to the more complete weathering of the soil. On the 

 other hand, the more soluble limestones rapidly concentrate their 

 siliceous materials to a soil rich in very fine feldspathic and other 

 particles.' 



In short, the daily and yearly action of the weather upon such soils 

 would not show a yield of alkali g-reater in the case of those residual 

 from ig-neous rocks than from those residual from sedimentary rocks. 

 The attack on the rock beneath must furnish a very minute supply of 

 alkalies contrasted with what is proceeding from the soil. Merrill 

 refers to a calculation in reference to one of the Trenton limestone soils 

 that in every cubic foot of soil "158,000 square feet of surface are 

 exposed to the action of water and air as well as to the roots of 

 growing- plant.'' ' 



It is, too, a fact of common observation and comment, that ig-neous 

 and eruptive rock masses are more slowly denuded than the majority 

 of sedimentaries. Whether in regions of limestone or slate the higher 

 and more abrupt surface features are generally the granitic or igneous 

 masses, and this obtains although the weathering, as dependent on 

 chemical decomposition, is most active on the eruptives.'' The etfect 

 is greatly physical in origin. 



"In stratified rocks there is as a rule a lack of homogeneity, cer- 

 tain layers being more porous than others or containing mineral con- 

 stituents more susceptible to the attacking forces."* 



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

 knowledge extends, appears in MerrilFs 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 resid- 

 ual 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 Paleozoic epoch, and which would have contributed not only more 



^See Rocks, Rock Weathering, and Soils, p. 307. 

 '•'Loc. cit., p. 308. 



^Merrill, loc. cit., p. 27L The familiar appearance of igneous dikes standing out 

 like walls above surrounding sedimentary rocks is an example. 

 *Loc. cit., p. 248. 



