434 LAMLASH. GEOLOGY. 



lash, although rendered invisible by the positions of the 

 beds with respect to the sea, which excludes whatever may 

 lie below the white strata. Fragments of conglomerate 

 are found scattered on the shore, indicating that some of 

 the beds possess this structure. Occasionally, the white 

 sandstone contains rounded pebbles of trap, and not 

 unfrequently, when in the immediate vicinity of the over- 

 lying rock, presents that indication of a crystalline ten- 

 dency which was formerly described as occurring in similar 

 situations near Blackwater bay. 



The sandstone beds now described are covered by a 

 mass of felspar rock which constitutes the chief part of the 

 island. Its greatest thickness may be estimated at 800 or 

 900 feet, taking it from the point of contact with the 

 sandstone to the summit of the mountain. This mass 

 appears, on a general view, to be disposed in regular beds 

 following these strata in a conformable manner; and, 

 without a careful examination, would be pronounced a 

 parallel and alternating rock. But on the eastern side, 

 it will be seen in one place to intersect the whole body 

 of strata; the intersecting portion being connected with 

 the more extensive overlying one, in the same manner as 

 it is in different parts of Arran. The sandstone is 

 in this place found abruptly terminating against the 

 former portion, yet the junction is not attended by any 

 peculiar disturbance. The accompanying sketch will 

 render this circumstance more intelligible than any 

 description. * 



In the lower parts of the mountain, the natural fracture 

 of this rock is vertical and prismatic, resembling that of 

 the obscurely columnar traps. Many faces, presenting 

 this character, and varying from 20 to 150 feet or more in 

 height, are visible on the eastern side of the island. On 

 the summit, it is found in a schistose form, similar to that 

 already described as occurring both in Mull and in Arran ; 



* Plate XXIV. fig, 2. 



