260 COMPOSITION OF SCHORL. 



■|th part of their whole weight. Such differences must materially af- 

 fect the constitution and agricultural capabilities of the soils formed from 

 these several minerals, and they show the correctness of what I have 

 previously stated to you — that mineraloi^^koai differences in rocks which 

 may be neglected by the geologist, may be of great importance in ex- 

 plaining the appearances that present thtaiiselves to the philosophical 

 agriculturist. 



4°. Schorl usually occurs in the form of long black needles or prism.s 

 disseminated through the granitic rock, and generally (in Cornwall) at 

 the outskirts of the granite, where it comes into contact with the slate 

 rocks that surround it (De la Beche). It consists of a silicate of alumi- 

 na in combination with silicates of iron and of soda or magnesia. Two 



varieties gave by analysis — 



Schorl Tourmaline 



from Devonshire. from Sweden. 



Silica, 35-20 37-65 



Alumina, .... 35-50 33-46 



Magnetic Oxide of Iron, 17-86 9-38 



Magnesia, .... 0-70 10-98 



Boracic Acid, . . . 4-11 3-83 



Soda 2-09 Soda & potash, 2-53 



Lime, 0-55 0-25 



Oxide of Manganese, 0-43 — 



96-44 98-08 



This mineral, according to these analyses, is characterised by con- 

 taining from J to ^ of its weight of magnetic oxide of iron,* and some- 

 times ~Q of magnesia. The presence of Boracic acidf is also a remark- 

 able character of this mineral, but as neither the presence of this sub- 

 stance in any soil, nor its effect upon vegetation, have hitherto been ob- 

 served, we can form no opinion in regard to its importance in an agri- 

 cultural point of view. 



§ 2. Q/* the degradation of the Granitic rocJcs, and of the soils formed 

 from them. 



The granites, in general, are hard and durable rocks, and but little af- 

 fected by the weather. The quartz they contain is scarcely acted upon at 

 all by atmospheric agents, and in very many cases the felspar, mica, 

 and hornblende yield with extreme slowness to their degrading power. It 

 is chiefly to the chemical decomposition of the felspar that the wearing 

 away of granite rocks is due, and the formation of a soil from their crum- 

 bling substance. 



It has been stated that the felspars consist of a silicate of alumina in 

 combination with silicates of potash or of soda. New these latter sili- 

 cates are slowly decomposed by the carbonic acid of the air (see p. 207), 

 which combines with the potash and soda, and forms carbonates of these 

 alkalies. These carbonates are very soluble in water, and are, there- 



• This oxide is composed of the^rK and second oxides of iron described in p. 210. 



T Boracic acid occurs in combination with soda in the common borax of the shops. It 

 combines wilh soda, potash, lime, &c., and forms borates. In tho schorl it probably exists 

 In such a slate of combination. 



