226 GENERAL REMARKS ON .GNEISS. 



number will be found to pass by insensible degrees into 

 the more regular kinds. 



The first division is subdivided into three parts, 

 each of them being characterized by a peculiarity of 

 structure. 



The granitic subdivision is that which prevails through- 

 out the islands now described, and it is characterized, 

 not only by a large grain and imperfectly foliated struc- 

 ture, but by frequent partial transitions into granite, from 

 which, when in detached specimens, it cannot sometimes 

 be distinguished. 



The second, or schistose subdivision, occurs principally 

 on the mainland of Scotland, in numerous situations ; 

 alternating with and graduating into micaceous schist 

 and into quartz rock ; in some cases so frequently, that 

 it is impossible, in describing or in mapping a given 

 district, to say under which head the rocks ought to be 

 enumerated. In its general aspect it is small grained ; 

 with a granular texture when connected with quartz rock, 

 and with a distinctly laminar one when it passes into 

 micaceous schist. The felspar can in many cases only 

 be distinguished from the quartz by its decomposition on 

 the weathered surfaces; and, when these are not acces- 

 sible, this gneiss is easily confounded with the one or the 

 other of the two rocks above mentioned. 



The last subdivision, or the laminar, seems to be of 

 more limited occurrence, but it may be seen in Rona, in 

 Sky, in Ardgower, and in other places. It is charac- 

 terized by the laminar alternating disposition of the ingre- 

 dients; the specimens often exhibiting in consequence 

 of this arrangement, singular and beautiful combinations 

 of colour. 



It will be perceived that, as in the case of granite, I have 

 here admitted hornblende together with mica as partici- 

 pating in the composition of regular gneiss. In fact this 

 mineral is more prevalent than mica in the gneiss of 

 the Western isles ; nor does there appear any reason 



