﻿Vol. 64.] STRFCTURE OF THE ST. DAVId's AREA. 381 



mode of occurrence of the vesicles, when they are carefully 

 examined. They are not characteristic of the edges of the basic 

 masses ; indeed, they rarely, if ever, occur near the margins, but rather 



Eig. 2. — Cambrian conglomerate involved in a basic intrusion ; 

 ivest of Carn-aru'ig^ Ramsey Sound. 



J. Barrow i)hotogr. 



[B— Basic intrusion; = Conglomerate.] 



some way within the intrusion. The best illustration of the latter 

 point occurs at Carn Howell, where small vesicles, usually about 

 3 millimetres in diameter, are very abundant, but at a distance 

 of at least 100 yards from the margin. 



VIII. Summary. 



The conclusions arrived at may be briefly summarized as 

 follows : — 



(1) The Pebidian consists of a bedded series of tuffs, composed of 

 detrital volcanic matter deposited under water. Although 

 some of the material may have been blown directly into its 

 present position, the bulk has been washed down from a 

 land-surface. 



(2) This has resulted in the formation of singularly persistent 

 bands of deposition, capable of being traced for considerable 

 distances and mapped out on the ground, the structure of 

 which has thus been determined. 



