62 COLL. GEOLOGY. 



specimens variously proportioned with regard to their 

 ingredients- succeeding each other in parallel and alter- 

 nating laminae : and since mica is often superadded to 

 the triple compound of felspar, quartz, and hornblende, 

 the various ways in which this mineral is disposed tends 

 still further to increase the number of varieties. 



Although the gneiss which is irregularly disposed, 

 resembles in composition that which displays a 

 parallel position and rectilinear direction, it is distin- 

 guished by some important peculiarities. It is every 

 where traversed by innumerable small reticulating veins of 

 granite, a circumstance so well known as a frequent 

 characteristic of gneiss, as to need no further de- 

 scription. It exhibits at the same time the most 

 capricious contortions. These reticulating veins occur 

 but rarely in that gneiss which is disposed in a regular 

 manner; or rather they are almost altogether absent. 

 Instead of them there are to be observed in this variety 

 of the rock, granitic laminae, which are parallel to the 

 position of the beds and of course alternate with the 

 real gneiss. They are easily distinguished by the total 

 absence of foliated tendency : in hand specimens no ques- 

 tion would remain respecting their place among the 

 granites. In many cases, however far they are traced, 

 they still appear to be uniformly parallel to the gneiss 

 and to avoid any interference with it; while in others 

 the parallelism is but temporary and they at length quit 

 their even course, becoming either thicker or thinner, or 

 cutting in upon the course of the laminae. Hence, 

 although we might, from the instances first quoted, 

 conceive them to be regular beds of granite alternating 

 with the gneiss, the occurrence of even one deviation of 

 this nature is sufficient to prove that they are disposed 

 in veins, of which the directions are parallel to its 

 laminae. It is easy to understand why the parallelism of 

 the gneiss is preserved where it is traversed by veins 

 of this nature, while its evenness is interrupted and 



