SOIL MATERIAL AND ITS ORIGIN 9 



tain masses of chert, impure quartz, or flint which may constitute 

 no small part of the soil, thus giving rise to cherty or llinty soils. 

 The rapidity with which a soil is formed from a limestone depends 



Fio. 1. Limestone composed chiefly of shells of Brachiopods. (Church.) 



Fro. 2. Limestone containing large amounts of Crinoid stems. (Church.) 



upon the amount of impurities present. A limestone containing 

 two per cent of impurities could leave approximately two feet of 

 residue for each 100 feet of limestone removed in solution provided 

 nothing is lost hy erosion. Limestone soils are usually fertile. 



