34 



SOILS. 



dant as a rock-ingredient, but it also the one most easily de- 

 composed and therefore most commonly concerned in soil 

 formation. The black hornblende owes its easy decomposi- 

 tion under the atmospheric influences to two properties; one, 

 its easy cleavage, whereby cracks are readily formed and ex- 

 tended by the agencies already mentioned (pp. 1-3). The other 

 is its large content of ferrous silicate (silicate of iron pro- 

 toxide), \vhereby it is liable to attack from atmospheric oxy- 

 gen; the latter forms ferric hydrate (iron rust) out of the pro- 

 toxide, thus causing an increase of bulk which tends to split the 

 masses of the mineral in several directions, while the silex is 

 set free. At the same time the carbonic acid of the air con- 

 verts the silicate of lime and magnesia, which forms the rest 

 of the mineral, into carbonates; and the alumina present forms 

 kaolinite, as in the case of the feldspars. There is thus 

 formed from this mineral, when alone, a strongly rust-colored, 

 more or less calcareous and magnesian clay, constituting the 

 material for rather light-textured '' red ' soils. In most 

 cases however the hornblende is associated in the rock itself 

 with the several feldspars, (mostly lime- and soda-lime feld- 

 spars) as well as with more or less quartz. The rust-colored 

 soils are therefore most commonly the joint result of the 

 weathering of these several minerals. This is well exempli- 

 fied in the case of the " red " soils formed from the so-called 

 granites and slates of the western slope of the Sierra Nevada 

 of California. 



Pyroxene or Augite so nearly resembles hornblende in its 

 chemical composition and crystalline form, that what is said of 

 the latter may be considered as applying to augite also. Owing 

 however to the absence of any prominent tendency to cleavage, 

 the smooth crystals of this mineral are attacked much less 

 readily than is hornblende, so that we often find them as 

 " black gravel ' in the soils formed from rocks containing 

 it. Such soils are particularly abundant and important in the 

 region covered by the great sheet of eruptive rocks (basalts, 

 so-called) in the Pacific Northwest, and on the plateau of 

 South Central India (the Deccan), and result likewise from 

 the decomposition of the black lavas of volcanoes; thus in the 

 Hawaiian islands, and in the Andes of Peru and Chile. 



Both hornblende and augite being either free from, or de- 



