EXCURSION IV. 101 



augite (with some kind of felspar) is the invariable in- 

 gredient in recent volcanic rocks. Hornblende usually 

 shews a slight play of two colours, augite has one uniform 

 aspect. Amphibole is a general name for hornblende and 

 its varieties, as actinolite, tremolite &c. ; Pyroxene for 

 angite and its varieties, as diallage, hypersthene, and 

 sahlite. It is important to bear in mind, when study- 

 ing these rocks in the field, that however different they may 

 appear, they are all very similar in composition, the differ- 

 ences among them being due to different rates of cooling from 

 a state of igneous fusion. Rapid cooling produces a glassy 

 substance like the obsidian or volcanic glass of modern 

 volcanic districts ; but if the fused matter cool slowly it 

 crystallizes, a complete physical change takes place, and a 

 stony structure is assumed. 



The chemical composition also of the greenstones and 

 basalts is very much alike, ranging from 45 to 60 per cent, of 

 silica, 10 to 20 per cent, of alumina, 10 to 15 and sometimes 

 25 per cent, of oxide of iron, with small quantities of potash, 

 soda, lime, and magnesia. It is chiefly the black basalt of Staffa 

 and the Giant's Causeway that contains so large a proportion 

 of iron. Other varieties of the hornblendic traps of Arran 

 are those improperly called trap-porphyries. They are fine 

 greenstones, containing imbedded crystals of felspar; and the 

 base being thus compound, they are not properly porphyries 

 One is speckled with felspar dots ; the other more homo- 

 geneous and dark-based, with distinct felspar crystals. 



Diorite, or common greenstone, is also found in spheroidal 

 masses, which are very interesting, as illustrating the origin of 

 these rocks. The spheroids decompose in concentric coats, and 

 are found to be finer-grained inwards, so that the nucleus is 

 often very fine in the grain. As the crust thickened by 

 cooling, the heat would part from the centre more and more 

 slowly, and the most perfect structure would be developed at 

 the centre. This structure is indeed singularly characteristic 

 of the trap rocks, the slightest development of it enabling us 



