476 ON THE CONNECTION OF GEOLOGY 



productive ; or, where rocks of a different nature alter- 

 nate with masses of pure limestone, having a greater 

 capability than it of contributiug to the formation of pro- 

 ductive soil. When water, containing carbonic acid, 

 passes through limestone rocks, it dissolves portions of 

 it, and deposits them in other places, by which the 

 decomposition of the limestone and the formation of loose 

 earth may be in some measure accelerated. 



To the third class belong chalk and gypsum ; which, 

 in so far as regards their decomposition by chemical 

 means, are of a similar nature with compact limestone ; 

 but possessing a much slighter cohesion of parts, are more 

 liable to be broken down by mechanical means. Water 

 also dissolves gypsum, and thus assists in its disintegra- 

 tion. The soil arising from these rocks resembles that 

 produced by compact limestone, which explains the want 

 of fertility, observable in certain gypseous tracts of the 

 North of Germany, and in the chalk districts of France. 

 The fertility which we see in certain places where chalk 

 is the fundamental rock, as in the Isle of Wight, Island 

 of Rugen, &c. is to be attributed as well to argillaceous 

 and marly strata alternating with the chalk, as to the 

 greater humidity of the atmosphere, by which the dry- 

 ness and heat of the soil are diminished. 



In the fourth class we place certain rocks, composed 

 of different minerals, but compact in appearance, which, 

 although they resist mechanical disintegration, are yet 

 subject to chemical action, and are, by means of it, con- 

 verted into a loose, compound productive soil. Of this 

 kind are basalt, and some other rocks very nearly allied 

 to it. 



To the fifth class we refer those rocks which have 



