LAM LAS II. GEOLOdV. 4.';, 



and although tlia combination of the prismatic and schis- 

 tose fractures is not so decided here as at Corygills, it is 

 nevertheless to be observed in different places. Wher- 

 ever it is found, the ends of the columns first divide into 

 imperfect laminre, and at length the mass loses the colum- 

 nar aspect and becomes continuously slaty. * 



The mineral characters of this rock are similar to those 

 of the corresponding substances in Arran, but offer rather 

 less variety. It passes from a clinkstone to a hard clay- 

 stone, often by a very gradual transition ; and is remark- 

 able for the great depth to which it is sometimes affected 

 by the action of the weather. This change consists chiefly 

 in the substitution of an arenaceous for a compact fracture, 

 and of an arid for a translucent surface ; being accom- 

 panied at the same time by a change of colour ; the grey 

 varieties becoming yellow, or white, or presenting different 

 successive zones of these tints. Nothing can be added 

 on this subject to the few remarks that were offered when 

 formerly treating of the Slate isles ; and I may conclude 

 the description by saying that it presents considerable 

 varieties of colour, namely, whitish, ochre yellow, dark 

 lead-blue, smoke grey, and purplish grey. 



It has been seen that the trap rocks occur in Arran, both 

 intermixed in distinct portions with the rocks of the felspar 

 division, and overlying them. None are found in Lamlash 

 occupying these situations ; all those which are here 

 visible, being either ordinary veins, or masses involved in 



* With respect to the schistose rocks of this family, whether simple 

 or porphyritic, it is proper here to remark, that in no instance which has 

 yet occurred in my observation, can the schistose division be effected by 

 force. Neither does it appear to be found in the deeper parts of the 

 rock ; being, on the contrary, limited to the surfaces, or to a small depth 

 beneath. It would therefore seem to be the result of an ineipient de- 

 composition, and, as remarked in Arran, is analogous to that which takes 

 place in certain varieties of granite. It is important to observe, that it is 

 very distinct in every respect from the schistose structure as it occurs in 

 the argillaceous slates. 



