Action of the Atmosphere upon newly -deepened Soil. 463 



The felspar seems to give way often more readily than even 

 horneblende ; thus I have in my possession specimens of syenitic 

 greenstone, struck off the outside of a large weather-worn boulder, 

 having the surface covered with protuberances of horneblende, 

 the felspar that originally lay between having been all decom- 

 posed and washed away by the rains ; and tracts of rotten granite, 

 as has been said, are of not unfrequent occurrence in this quarter, 

 decayed into an earthy gravel, apparently by the loosening of 

 the felspathic paste in which the crystals of quartz are imbedded. 

 This stuff is much in request for garden walks, for which it is 

 often well suited by its whiteness. 



Mica. — The most common kind is the potash mica. Its chemi- 

 cal composition is very variable, but, according to L. Gmelin, 

 approaches to three parts of silicate of alumina with one of sili- 

 cate of potash, part of the potash being occasionally replaced by 

 protoxide of iron or manganese, and the alumina by the peroxide 

 of these metals. The lithia and magnesia micas are much less 

 frequent. Mica is a common ingredient in granite, gneiss, and 

 many slates and crystalline rocks, and is one of the most generally 

 distributed minerals. It seems to decompose slowly, apparently 

 owing to the small proportion of alkali it contains. Dr. Paris has 

 remarked that the relative proportion of the mica would seem to 

 have an effect upon the character of the soils derived from the 

 granite in Cornwall, tending to render them less fertile, an observa- 

 tion which is confirmed by De la Beche. The action of the 

 atmosphere upon it is apparently similar to what takes place in 

 felspar, the ultimate result being a clay, the potash or magnesia 

 dissolving out. 



Of these three minerals — quartz, felspar, and mica — are com- 

 posed all the granites, gneiss, mica-slate, quartz-rock, and many 

 sandstones, conglomerates, and porphyries. Common granite is 

 composed of — 



Quartz 40' 



Orthoclase felspar 40" 



Mica 20- 



100- 



— the proportions in gneiss being somewhat similar. Mica-slate 

 is oftcni almost wholly composed of mica, or of that substance and 

 quartz ; while quartz-rock and many sandstones are little else but 

 impure yarioties of quartz. 



Chlc.ritc, Talc, Steatite, Serpentine. — These are all members of 

 the mica family, are abundant, widely- dispersed minerals, and 

 agree in containing a large proportion of magnesia. 



Chlorite is frec|ucntly found in granite, gneiss, porphyries, and 



2 I 2 



