ARRAN. GEOLOGY. OVERLYING ROCKS. 599 



the compact felspar, or clinkstone, the one of a dark, the 

 other of a light colour. In the former case, the addi- 

 tion of the hornblende to the basis of clinkstone or com- 

 pact felspar, is so gradual, that the change is at first 

 not perceptible, and specimens of the two rocks are 

 therefore easily confounded. This also results in other 

 cases where the quantity of the additional ingredient 

 is considerable; the dark colour of the compact felspar 

 concealing that of the hornblende. 



As it is often impossible to assign names to these tran- 

 sitions, it is not surprising if different observers disagree 

 in describing these rocks, by referring them to one or 

 other of the extremes. But the transition from the 

 white or pale varieties, is more defined as well as more 

 obvious ; and the specimens are often interesting from 

 the beautiful manner in which the hornblende crystals 

 are disposed in the basis or intermixed with it. Some- 

 times they appear in the form of small obscure spots, 

 as in the rock of Ailsa; in which case if quartz be also 

 conjoined to the felspar in the basis, the rock will be 

 a syenite instead of a greenstone. In another case they 

 are disposed in an arborescent manner, in a third in 

 long distinct prismatic crystals ; while the gradual increase 

 of the hornblende at length produces greenstones of a 

 more ordinary aspect, which are, in a further progress, 

 to terminate in basalt. These several compounds vary 

 much in the magnitude of their crystallization, and in one 

 part of the island, (not far from Glen leg) three distinct 

 kinds are found aggregated together in one rock ; in 

 that respect much resembling those varieties of granite, 

 described in the beginning of this account, which contain 

 line grained concretions imbedded in a coarser basis. A 

 variety still more singular occurs in the same place. This 

 consists of a mixture of two distinctly coloured greenstones, 

 the one nearly white, and the other nearly black, from 

 the different proportions of hornblende in them; being 



