﻿Yol. 64.] STETJCTDEE OF THE ST. DA.VID's AREA. 365 



when underlying pasturage or paths, and is consequently easy to 

 trace and to map with exactness, where not obscured by drift. 



The finest exposure is that in the Caerbwdy Valley, where it 

 attains a thickness of 70 feet : usually it is about 40 feet thick. 

 jS'ormal junctions with the Pebidian are shown east and west of 

 St. Non's Bay, at Maen Bachau, and at Whitesand Bay. The 

 conglomerate has been most assiduously searched by investigators 

 for fragments of the Pebidian and Dimetian. Subangular pebbles 

 comparable with the former are common, but no undoubted coherent 

 fragment of the latter has been found. 



(2) Green Sandstone, — A fine-grained, well-bedded rock, 

 weathering ochreous where moist. In the specimens that I have 

 examined, the grains are of uniform size, measuring about •! to '2 

 millimetre in diameter, and principally quartz. Felspar is fairly 

 plentiful, and there are also grains of felsite, palagonite, clastic 

 biotite, ilmenite, and ferruginous spherulites. 



The matrix is plentiful, mainly of chlorite associated with minute 

 particles of various minerals, such as epidote, and, where crushed, 

 sericite. With such a composition the rock might be expected to 

 be sensitive to thermal metamorphism. 



The usual thickness of the Green Sandstone is from 400 to 500 

 feet, but near Porth-Clais it thins considerably, down to between 

 150 and 200 feet. Some of the diminution may be owing to com- 

 pression, as it is more squeezed in this district than elsewhere. 



(3) Red fossiliferous Shales, with Liyigulella primceva^ 

 etc. — These beds are usually not more than 50 feet thick, and 

 have been put with the next group in the maps. 



(4) Purple Sandstone. — This is usually about 900 feet thick, 

 and near the top becomes a coarse grit, with small granitoid pebbles 

 that have been compared to the Dimetian.^ 



[b) The Solva Beds. 



The Lower Solva Beds consist of 150 feet of yellow grits and 

 flags ; the Middle and Upper Solva of 1500 feet of grey or greenish 

 flags and sandstones, with a purple band about one-third way up 

 the series. 



[c) The Menevian. 



This division consists of about 600 feet of black shale. 



The foregoing succession is substantially that recorded by the late 

 Dr. Hicks. 



The detailed mapping of these Cambrian rocks reveals a system 

 of faults, most of which have their general direction either from 

 north-north-east to south -south-west or from east to west. These 



1 T. a. Bonney, Quart. Journ. Geol. Soc. vol. xlii (1886) p. 358. 



