JoLY — An Estimate of the Geological Age of the Earth. 57 



residual material from the most diverse rocks are very similar in compo- 

 sition. 



The full tables should be consulted.* 



In the above it appears that the soils derived from the igneous rocks — more 

 especially the more basic ones — show a greater poverty in alkalies than those 

 derived from Limestones and Sandstones. This probably arises, in part, form 

 more soluble alkali constituents being present, but in many 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 felspathic and other particles. f 



In short, the daily and yearly action of the weather upon such soils would not 

 show a yield of alkali greater in the case of those residual from igneous 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 sur- 

 face are exposed to the action of water and air as well as to the roots of growing 

 plants. "J 



It is, too, a fact of common observation and comment, that igneous and 

 eruptive rock masses are more slowly denuded than the majority of sediment- 

 aries. Whether in regions of Limestone or Slate the higher and more abrupt 

 surface features are generally the granitic or igneous masses ; and this obtains 



* Loc. cit., pp. 305, 306 ; also, pp. 358, 359. 



t Zoc. cit., p. 307. 



I Zoc. cit., p. 308. 



K2 



