120 Geological Notes from Arran. [Sess. 



position would have approximated more or less to that of the 

 rocks among which they occur. This, however, does not appear 

 to be the case. The dykes are easily recognised as of igneous 

 origin, and do not show any particular difference even at their 

 surface of contact with the older rocks among which they are 

 found. Their intrusive character is easily established by the 

 fact that they have hardened, in a more or less degree, the 

 sandstones and conglomerates in their immediate vicinity. 

 Most of the dykes are composed of a kind of whin or basalt, 

 of such a nature that it is more susceptible to the action of the 

 weather and other agents than the surrounding rocks. The 

 consequence is the dykes have in a great measure disappeared, 

 and their position is indicated by more or less wide fissures in 

 the hard sandstones and conglomerates, most of them running 

 in fairly straight lines, and imparting a striking appearance to 

 the rocky platform. I have two specimens of these dykes, one 

 of the basalt variety and the other of quartz felsite. The 

 dykes of the latter material still stand rather higher than the 

 rocks among which they are found. It is curious to observe 

 the different degree of action on the surrounding rocks effected 

 by these dykes at the time when they were intruded in a 

 molten condition. No rule can be laid down ; many of the 

 more insignificant dykes appear to have had a greater effect in 

 hardening the rocks with which they are in contact than 

 some of the more important ones. I have some specimens 

 of hardened sandstone and conglomerate collected in the 

 immediate neighbourhood of two dykes. Some of the 

 dykes that run across the shore inland may be traced up 

 the face of the cliffs to a great height, showing that the now 

 low platform of the shore must at one time have had an equal 

 elevation. 



At some little distance past Corrygills, towards Clauchlands 

 Point, there is to be seen in the face of the sandstone cliffs a 

 large vein of that peculiar igneous rock pitchstone, which runs 

 in a nearly horizontal direction. The rock has assumed a pris- 

 matic structure on cooling, the prisms standing nearly vertical 

 and in close contact. It is an extraordinary vein, and looks 

 on close view almost like a wall of dark bottle-glass, about 

 10 feet high. I secured a good specimen in situ, and also a 

 fragment of the sandstone in immediate contact below, which, 



