THE VARIOUS ROCKS AS SOIL-FORMERS. 5 i 



called, a good many are not " true to name " and therefore 

 form soils differing materially from the type just mentioned. 



Thus the so-called granite areas of the Sierra Nevada of California 

 are largely occupied by a rock containing, besides quartz, chiefly soda- 

 lime feldspar and some hornblende, and scarcely any mica. It is more 

 properly a diorite (grano-diorite) ; the soils formed from it are rather 

 poor in potash, not strongly calcareous, and quite poor in phosphoric 

 acid. On account of the small proportion of hornblende (unusual in 

 diorites), these soils are light-colored (not " red " ), and bear a growth 

 of small pine instead of the usual oak growth of the lower Sierra 

 slopes. 



What is said of granite soils is also generally true of those 

 formed from 



Gneiss, which is composed of the same minerals as granite, 

 but has a slaty cleavage and on that account when upturned on 

 edge, weathers rather more rapidly than most granites. 

 Owing to the frequent occurrence of lenticular masses of quartz 

 in gneiss, its soils are more commonly of a siliceous nature 

 than are those of the true granite regions, and not as " strong " 

 as the latter. This is the more true since gneiss often passes 

 gradually into mica schist, which, being a mixture of quartz 

 and mica only, not only weathers very slowly but also supplies 

 but little of any importance to plants, to the soils formed 

 from it. Such soils would mostly be absolutely barren but for 

 the frequent occurrence in the rock, of accessory minerals that 

 yield some substance to the soil. Yet it remains true that in- 

 asmuch as gneiss and mica-schists are among the rocks in 

 which mineral veins most commonly occur, the proverbial 

 barrenness of mining districts is very frequently traceable to 

 these rocks. The same may be said of some of the related 

 rocks, such as gabbro, minette and others. 



Normal diorite consists of hornblende and soda-feldspar, 

 with more or less quartz. 



The soils derived from certain diorites of the Sierra Nevada 

 of California have just been referred to. Rut these granite- 

 like diorites are on the whole exceptional ; it should be added 

 that the (diabasic) " greenstones " of the Eastern United 

 States and of the Old World, which are usuallv much finer- 



