LUIXG, GEOLOGY. 141 



curvatures are infinitely more complex ; while the beds 

 appear also to be similarly affected throughout: besides 

 which there occur certain minute circumstances connected 

 with the contortion, apparently depending solely on me- 

 chanical disturbance. The decision of this question must 

 depend in a great measure on our increasing knowledge 

 of this rock, of which, common as it is, the geological 

 history is but imperfectly understood. It is evident 

 that the leading doubt in this case respects the structure 

 of this substance ; namely, whether the position of the 

 mica is the result of crystallization or deposition, a 

 question which I shall immediately have occasion to 

 notice. I need scarcely add, that the contortions of gneiss 

 are, to a certain extent, involved in the same decision; 

 although in former parts of this work, undeniable instances 

 have been adduced of contortions effected by the passage 

 of granite veins. 



The same trap veins which have already been described 

 in Seil and in Torsa, occur in Luing, and in as great 

 number and variety. They consist of basalt, common 

 greenstone, claystone, trap porphyry, and compact felspar; 

 and their positions, as in those islands, are sometimes 

 parallel to the strata, at others transverse. To describe 

 these particularly would be to repeat what has already 

 been said, yet two of them are deserving of notice. 



One of these occurs at the southern extremity of the 

 island, and consists of a lax arenaceous claystone exactly 

 similar to that rock as it is seen in Arran in overlying 

 masses ; a substance well known to geologists. As far 

 as I could penetrate, its texture is uniform throughout; 

 yet, from an analogous occurrence in Torsa, there appears 

 reason to suspect that it is a weathered or partially de- 

 composed rock. 



The other vein is, to a certain degree, parallel to the 

 beds in which it lies, and may be seen near the north- 

 eastern angle of the island. It is composed of a brownish 

 grey claystone, containing numerous dispersed scales of 



