202 COMPOSITION OF DIFFERENT ROCKS. 



much alumina, and very considerable quantities of 

 potash or soda. There is scarcely any lime, and no 

 phosphates, beyond perhaps mere traces. Some varie- 

 ties of granite do contain these substances in fair 

 proportions, but for the most part there is very little 

 of either. Hence granitic soils are frequently cold 

 and poor, particularly on the sides of the hills. In 

 the valleys they are apt to be better, as the best pari 

 of the decomposing minerals naturally washes down 

 the slopes. The abundance of alumina, however, 

 often makes these soils quite stiff. 



The true trap rocks, or basaltic rocks, also contain 

 feldspar, but with it an abundance of another mineral 

 called hornblende; or another still, called augite. 

 Both of these abound in lime, and consequently in 

 this class of rocks, according to theory, we have the 

 materials for producing soils superior to those formed 

 on the granitic regions. Practice supports the same 

 view; the greenstone traps and basalts almost inva- 

 riably form strong, good soils, fitted for the successful 

 cultivation of almost any crop. Some of the richest 

 land in Scotland is on this formation. 



The trap rocks vary in different situations, as to 

 their proportion of lime. In nine samples examined by 

 Prof. Johnston, the percentage of lime ranged from 

 2 to more than 10 per cent. These soils are so rich in 

 some places, that the surface is carted away to spread 

 upon poorer fields. 



The same differences of composition occur among 

 the stratified rocks. Some form very excellent soils, 

 and others very barren ones. The annexed diagram 

 will show how the soils alter, from what is called the 

 cropping out of mineral strata. 



Fig. II. 



