50 SOILS. 



the predominant mineral or minerals of compound rocks de- 

 termine the facility of weathering, as well as the quality of the 

 soil resulting therefrom. Since rocks are named essentially 

 in accordance with the kinds of minerals that constitute their 

 regular mass, the proportion in which the several constituents 

 stand to each other may vary greatly. Thus a granite may 

 consist, over considerable areas, mainly of a mixture of potash 

 feldspar and quartz ; in others, mainly of quartz and mica with 

 little feldspar. Very frequently, hornblende replaces mica par- 

 tially or wholly. The latter will weather much more slowly 

 than feldspar or hornblende, and will produce an inferior soil 

 when decomposed. Allowing for such variations, a fairly ap- 

 proximate general estimate of the quality and peculiarities of 

 soils from crystalline rocks may nevertheless be made. To 

 some extent such estimates must make allowance not only for 

 the chief ingredients, but also for those which are called " ac- 

 cessory ' or characteristic, and which while not present in 

 large amount, may nevertheless exert a considerable influence 

 upon the quality of the soil. 



Soils from granitic and crystalline rocks. In the case of 

 the (potash-feldspar) granite soils it is generally admissible 

 to expect that they will be fairly supplied with phosphoric 

 acid, because in the great majority of cases, minute crystals of 

 apatite (phosphate of lime) are more or less abundantly scat- 

 tered through it. From the potash feldspar present, granite 

 soils may always be relied on for a good supply of potash for 

 plant use; on the other hand, unless hornblende be present, 

 they are pretty certain to be deficient in lime, since neither 

 lime, feldspar nor calcite are probable accessory ingredients 

 of this rock. 



Granite is exceedingly apt to weather by mechanical disin- 

 tegration far in advance of its chemical decomposition. It is 

 therefore common to find in sedentary soils overlying granite, 

 a gradual increase of grains of its component crystalline min- 

 erals as we descend in the subsoil ; until finally the latter grades 

 off into rock almost unchanged save in lacking coherence. 

 This is seen strikingly in the southern Appalachians, as well as 

 in the Sierra Nevada and Sierra Madre of California; at Cin- 

 tra in Portugal, at Heidelberg in Germany, and elsewhere. 



But of the rocks that resemble granite and are popularly so 



