ARRAN. GEOLOGY. OVERLYING ROCKS. 409 



I believe be traced with any certainty. The last of these 

 overlying masses to be described, is conspicuous for its 

 limited extent, as the former is for the great space which 

 it occupies. It is moreover remarkable, as being the 

 northernmost overlying mass occurring in the island ; lying 

 in fact immediately on the boundary of that line which 

 was formerly described as separating the granite division 

 of Arran from that in which the trap rocks form the lead- 

 ing character. It is seen on the brow of the hill over the 

 head of Glen Shira, where that valley joins to Glen Rossie ; 

 lying above the sandstone and conglomerate, and not far 

 from the place where these terminate to be succeeded by 

 the micaceous schist. The area it occupies does not seem 

 to exceed fifty yards in breadth, but it offers no other 

 peculiarity requiring a further description. 



It is unnecessary to say that Arran presents instances 

 where the trap rocks are found in the same double con- 

 nexion with the secondary strata as the felspar rocks are ; 

 since this fact is of universal occurrence ; it is more neces- 

 sary to inquire respecting the mutual connexion of these 

 two classes of rocks. They pass into each other by insen- 

 sible gradations, as they have formerly been shown to 

 do in Rum. It is therefore unnecessary to inquire into 

 the relative antiquity of these two sets of rocks ; since 

 we are thus entitled to conclude that they are of the 

 same age, and that they owe their present position to 

 a common cause, or at least to a series of common causes. 

 For, although the circumstances now related, assign a 

 common origin to the two divisions, it is not necessary 

 that this should hold good of all the individuals. Appear- 

 ances too well known among the trap rocks to require 

 enumeration, and noticed in other parts of this work, 

 show that there have been successive formations of 

 them ; since they are found in the shape of veins inter- 

 fering with each other. There are not the same means 

 of demonstrating a successive formation of masses, be- 

 cause, as formerly observed, the superiority of such masses 



