52 SOILS. 



grained, do not form the mass of fine, angular debris consti- 

 tuting the subsoil in the Sierra Nevada, but weather into 

 rounded masses and fine-grained soils possessing, on the whole, 

 a fair fertility, though liable to contain an excessive proportion 

 of silex in various forms. 



Of the eruptive rocks as a class it is often said that they form 

 very productive soils ; yet, as these rocks differ widely from 

 each other in composition, this statement must be taken with a 

 great deal of allowance. Very many of them decompose with 

 extreme slow r ness on account of their glassy nature ; this is par- 

 ticularly true of obsidian, pumice stone, and the " volcanic ash " 

 derived from its pulverization, and which is found unchanged, 

 in sharp scales, among the decayed minerals of other rocks in 

 complex soils. Other volcanic ash, however, being formed by 

 the pulverization of crystalline or of basic lavas, weathers 

 rather readily, as already stated; so that certain plants take 

 possession in the course of a few years. The general classifi- 

 cation into basic and acidic rocks, given above, is of importance 

 in connection with soil formation from eruptive masses; for 

 the basic rocks are much more easily attacked by the atmos- 

 pheric agencies than the acidic class. 



A broad distinction must, however, be made between the basic rocks 

 of the basaltic class, which contain black pyroxene as a prominent 

 ingredient, and those which, like many trachytes, are rich in feldspr thic 

 minerals. The latter are naturally rich in r.lkalies (potash and soda) 

 which they impart to the corresponding light-colored soils ; while the 

 black basaltic rocks and lavas weather into " red " soils, sometimes 

 containing extraordinary amounts of iron (ferric hydrate) and (from the 

 lime-feldspars they contain) a fair supply of lime, but oftentimes very 

 little potash. Experience seems to prove that the red basalt soils are 

 mostly rather rich in phosphoric acid ; this is especially true of the 

 country covered by the great eruptive sheet of the Pacific Northwest, 

 in the rocks of which the microscope readily detects the presence of 

 numerous needles of apatite (lime phosphate). The same is true of 

 the highly iron-bearing soils from the black basaltic lavas of the Hawaiian 

 islands, even though they have been leached of all but traces of lime 

 and potash. All these soils are physically " light " and easily workable, 

 since the rocks in question contain but little alumina from which to 

 form clay ; they are sometimes extremely rich in iron, even to the 

 extent of being capable of serving as iron ores. 



