100 GEOLOGY OF ARRAN. 



trappa, a stair. They are also called Plutonic rocks, as 

 formed in the " nether depths," to distinguish them from the 

 volcanic rocks, erupted at the surface. 



The name Diorite is now usually applied to the rock 

 formerly called common greenstone, which consists of horn- 

 blende, and felspar usually of the variety called oligoclase; 

 Diallage rock and Hypersthene rock are the varieties in which 

 Diallage and Hypersthene take the place of hornblende. 

 Basalt is a nearly homogeneous compound of augite and fel- 

 spar, of a black or dark-blue colour, and great specific 

 gravity, owing mainly to the presence of 14 percent, of the 

 oxides of iron and manganese, and has several varieties 

 denoting merely difference of texture. Thus a coarsely 

 crystallized variety, in which the ingredients are distinct, is 

 called Dolerite, as it has then a deceitful resemblance to 

 diorite ; but if close grained, though still distinctly and 

 uniformly crystalline, is called Anamesite or intermediate; 

 basalt proper is compact and semi-vitreous, and has a perfect 

 conchoidal fracture. The composition of the fine varieties is 

 known, independently of analysis, mainly by the gradual 

 passage of coarser kinds into them. Ambiguity is best 

 avoided in naming these different varieties, by stating the 

 mineralogical composition and the grain. "We fail to see 

 the advantage, so far at least as British geology is concerned, 

 of the new names as above explained. Common greenstone, 

 the prevailing variety of those traps which consist of felspar 

 and hornblende, is quite as expressive as diorite ; and in- 

 stead of dolerite we have a name quite definite in the term 

 coarse basalt. But in truth the varying constitution of the 

 rocks defies all powers of nomenclature. Thus common green- 

 stone or diorite sometimes contains both augite and olivine, 

 as well as hornblende ; and in basalt we sometimes find 

 hornblende and olivine. But usually hornblende and augite 

 do not occur together; and augite may be regarded as a 

 fused and re-cooled hornblende, since the latter, when fused, 

 cannot be re-formed, but takes the form of augite, and since 



