THE TRAP-ROCKS GREEN-ST()>E. 263 



wrhile m aid of both these causes, a small diflTerence in the composition 

 of its constituent minerals, often not to be detected by the eye, may ma- 

 terially affect the character of the granitic soils. 



According to Dr. Paris, the presence of much mica deteriorates these 

 soils; while that which is formed at the edges of the granite, when it 

 comes in contact with the slate rocks, is of a more fertile quality. The 

 latter remark, however, does not universally apply, — especially where 

 the granite, as at the edges of Dartmoor, contains much scherl, (De La 

 Beche) — and the presence of mica, in the richest soils of the red marl, 

 would seem to imply that this mineral is fitted materially to promote 

 the fertility of a soil in which the other earthy ingredients are properly 

 adjusted. 



The more elevated and thin granitic soils are said to be fitted for the 

 growth of larch ; the lower and deeper soils, which admit of the use of 

 the plough, have been found to yield a three-fold return of corn by the 

 use of lime alone. 



§ 4. Of the trap rocks, and the soils formed from them. 



Of the trap rocks there are several varieties, of which the most impor- 

 tant are distinguished by the names of Greenstone, Basalt, and Ser- 

 pentine. 



The Green-stones consist of a mixture more or less intimate of felspar 

 and hornblende, or of felspar and augite. They are distinguished from 

 the granites by the absence of mica and quartz, and by the presence of 

 the hornblende or augite, often in eijual, and not unfrequently in greater 

 quantity than the felspar. In the granites, the felspar and quartz to- 

 gether generally form upwards of y^^ of the whole mass. 



Augite is a mineral having much resemblance to hornblende, and, 

 like it, occurring of various colours. In the trap rocks it is usually of a 

 dark green approaching to black. It generally contains much lime and 

 oxide of iron in the stale of silicates. The composition of two varieties 

 compared with that of basaltic hornblende is as follows : — 



Black Augite Augite from the Basaltic 



from Sweden. lava of Vesuvius. Hornblende. 



Silica 63-36 50-90 42-24 



Lime 22-19 22-96 12-24 



Magnesia 4-99 14-43 13-74 



P rot-Oxide of Iron . . . 17-38 6-25 ]4-69 



Prot-Oxide of Manganese . 0-09 — 0-33 



Alumina — 5-37 13-92 



9S-01 99-91 97-06 



The predominance of this mineral (augite) or of hornblende in the 

 green-stone rocks must necessarily cause a very material difference in 

 the nature of the soils produced from their decay, compared with those 

 which are formed from the granitic rocks in which the felspars are the 

 predominating mineral ingredient. 



2°. Basalt consists of a mixture, in variable proportions, of augite, 

 magnetic oxide of iron, and zeolite.* It differs in appearance from green- 



* " Wi7A or without felspar." In addition to augite, magnetic iron, and zeolite, many ba* 

 salts oontain also a considerable portion of certain varieties of felspar, especially of ODO to 

 which the name oinepheline has been given. 



