478 ON THE CONNECTION OF GEOLOGY 



portion of felspar which enters into its composition, it 

 does not afford so productive a soil. 



In the sixth class may be placed the slaty rocks, whe- 

 ther simple, or intimately compounded, which do not 

 readily undergo chemical decomposition, but which easi- 

 ly separate at their natural fissures, and are mechanically 

 resolved into an earthy mass, forming a paste with wa- 

 ter, circumstances which are observed chiefly in day- 

 slate, a rock of much importance in the formation of 

 productive soil, usually passing into a clayey sort of 

 earth. 



To the seventh class belong the conglomerated rocks, 

 whose parts indeed undergo very little, if any, chemical 

 change, but are easily separated by mechanical means, 

 and are thus converted into a gravelly, sandy, or earthy 

 mass. Of this kind are greywacke, old red sandstone ', 

 and sandstones of various kinds. Much diversity is ex- 

 hibited by these rocks, with regard to the facility with 

 which they undergo disintegration, as well as the nature 

 of the soil arising from them; circumstances which 

 chiefly depend upon the nature of the cement, and its 

 relation to the parts cemented. The disintegration of 

 these rocks is the more easily effected that the cement is 

 abundant, and less intimately connected with the other 

 parts, that is, the more they depart from a crystalline 

 nature ; on which account greywacke is less easily con- 

 verted into soil, than the common varieties of sandstone. 

 By the decomposition of greywacke, a loose and fertile 

 soil is formed, containing particles of quartz and clay in 

 due proportion ; on the other hand, by the decomposi- 

 tion of red sandstone, a soil is frequently produced, a- 

 bounding in argillaceous particles impregnated with iron, 



