64 COLL. MINERALS. 



describing Tirey, to renew the question respecting the 

 contemporaneous nature of the granite veins of gneiss. 

 But it is quite obvious throughout Coll, wherever the beds 

 possess sufficient regularity to render that circumstance 

 perceptible, that they are shifted by the passage of those 

 veins. I have given^two examples of this nature in a 

 diagram*. 



An equally remarkable instance of apparently mechani- 

 cal arrangement occurs at the southern end of the island, 

 of which there is also a representation subjoined f- 

 Analogous appearances must be familiar to all those who 

 have had opportunities of examining rocks of this 

 nature ; but the present example is interesting chiefly 

 on account of its magnitude and of the apparent 

 instance of softening and flexion which it affords. 

 The nodules of hornblende rock here represented are of 

 considerable size, attaining a diameter of three or four 

 inches, and the laminae of gneiss are every where bent 

 over them, as if they had, when in a soft state, been 

 forced by external pressure to accommodate themselves 

 to the previously indurated concretions. 



Such are the principal geological features of the gneiss 

 of Coll. I shall not here attempt to draw any further 

 general conclusions from the appearances which have been 

 described, since a considerable tract of the same rock re- 

 mains to be examined. If any useful views are to be de- 

 duced from the comparison of facts, they will be more 

 easily and more certainly established when a greater body 

 of materials shall have been accumulated. 



Since mica is an ingredient of gneiss, it cannot strictly 

 be considered an adventitious mineral ; yet it is here found 

 in concretions so large as to require notice among the 

 mineral substances to be enumerated. It is always black, 

 at times crystallized, and in other cases disposed in 

 large plates without definite forms. It is very often mixed 



* PI. xi. fig. 1, 2. t PI- ** v - % * 



