Action of the Atmosphere upon newly -deepened Soil. 465 



found in many slates, and indeed in all sorts of rocks, being' often 

 seen in its crystallized form of yellow cubes in common roofing- 

 slate. 



Hornehlende, Augite, Diallage. — These minerals are all of the 

 same family. Their chemical composition is very variable, but 

 they may be viewed as silicates of magnesia and lime, these bases 

 being I'eplaced often to a considerable extent by protoxide of iron, 

 and to a less degree by alumina and oxide of manganese. Horne- 

 blende generally contains more silica than augite ; the latter 

 mineral has also been thought not to occur in rocks containing 

 free quartz or silica, whereas horneblende does. Some mineralo- 

 gists and geologists, therefore, would make two distinct divisions 

 of rocks : — 1, the Horneblende series, including syenite, diorite, 

 and dioritic and red porphyries ; 2, the Augite series, compre- 

 hending hypersthene rock, dolerite, nepheline, and augitic and 

 leucite porphyries. The very near affinity, however, of these two 

 minerals is shown by some experiments of Gustavus Rose, who, 

 on fusing horneblende in a porcelain furnace, found that on cooling 

 it assumed the appearance of augite ; and hence some eminent 

 mineralogists favour the opinion that the distinction between the 

 two bodies arises merely from different modes of cooling, horne- 

 blende being the result of slow, and augite of rapid, cooling. 

 Diallage and hypersthene may be considered merely as varieties 

 of augite. 



Most of the great series of igneous and volcanic rocks are 

 composed of various combinations of felspar and horneblende or 

 augite, with occasionally some quartz, calcareous spar, and zeolites. 



The horneblendic minerals differ from the felspars and micas 

 generally by the large proportion of lime and oxide of iron tliey 

 contain, and by the absence of potash and soda. The igneous 

 rocks, therefore, which contain both felspar and hornelilende, will, 

 by their decomposition, form a much more fertile soil than what 

 would arise from granite, gneiss, &c., which generally contain 

 only one of these minerals ; for the felspar will supply the potash 

 or soda, in which the horneblende; family is deficient, wliile the 

 latter will contribute the lime, magnesia, and oxide of iron, which 

 are for the most part wanting in common felspar. 



Tlie horneblende minerals are amongst the most abundant in- 

 gredients of our rocks. i hey are very liable to be acted upon by 

 atmospheric influences, chiefly ttirough the oxidation of the prot- 

 oxide oi. iron, wliicli they so abundantly contain. In tlie case of 

 common horneblende, a rusty-brown powder gathers upon the 

 surface in consequence of the peroxide of iron which is formed by 

 the prott)xide combining with the oxygen of the air, and eventually 

 the mineral falls down into a brown ferruginous earth, the mag- 

 nesia and lime being liberated while water is absorbed, and a 



