﻿Vol. 64.] STKUCTUKE OF THE ST. DAVID's AREA. 371 



drawn between the two, but the change is sufficiently rapid for 

 field-mapping. The included bits of halleflinta and patches of 

 chlorite do not as a rule measure more than 4 or 5 millimetres in 

 diameter. It is well exposed in the Caerbwdy Yalley and on Carn 

 Gwil Geli, north of Treginnis-isaf. The rock becomes finer at the 

 top, where it passes into bedded blue halleflinta (C 4). 



The finest section of C 4 is again in the Caerbwdy Yalley, where 

 it has been quarried and is at least 90 feet thick. It is here an 

 evenly-stratified, blue, bluish-green, or green rock with bands of 

 yellow, minutely-porous stone, all breaking with a conchoidal 

 fracture. The blue and green stone is translucent in thin splinters, 

 and often looks like bottle-glass. Thin bands occur of coarser 

 felspathic rock. Microscopically the rock is of the usual Caerbwdy 

 type, but with little chlorite and only rare felspar-crystals. This 

 bed can be easily traced, on account both of its lithological 

 characters and of the clearness of the exposures, and consequently 

 is an important clue in working out the details of the structure. 

 From the Caerbwdy Yalley it can be followed north-eastwards up 

 to the main road, and south-westwards to the cliff's of St. Non's 

 Bay, of which it forms a considerable part. West of St. David's it 

 is again exposed 150 yards west of Treginnis-isaf and on Carn Each, 

 both localities being within 200 yards of the cliffs of Eamsey Sound. 



Above the halleflinta just described comes a thick group of 

 felspathic and porcellanitic rocks (C 5), much like C 3 in general 

 appearance, but often distinguishable by the absence of the frag- 

 ments of green halleflinta usually seen in the lower division. The 

 most typical rock has a peculiar ' pepper-and-salt ' appearance, due 

 to the presence of multitudes of greenish-black specks set in a pale 

 felspathic ground. The specks are shown by the microscope to be 

 patches of chlorite or epidote, in a ground-mass of normal Caerbwdy 

 type. Here and there a few bands occur in which quartz-grains 

 (of igneous origin) are sufficiently numerous to give a gritty aspect 

 to the rock. 



(D) The Ramsey-Sound Series. 



This series is characterized by the presence of a considerable 

 amount of sericite due to shearing, and by the comparative rarity 

 of fragments of lava or older tuffs. The rocks are usually rather 

 soft and thoroughly schistose. 



The lowest division (D 1) is a sheared blue (occasionally yellowish- 

 white) rock, sometimes exhibiting grains of quartz and felspar, but 

 otherwise without recognizable fragments. Often the shearing has 

 taken place along irregular surfaces, breaking the rock up into 

 rugose lenticles ; but more frequently, especially in the northern 

 part of the area, it is finely fissile, indeed almost papery. Thin 

 sections show that it is closely allied to the Caerbwdy Series, and is 

 indeed but a slight modification of the same material, the only im- 

 portant diflference being the presence in some specimens of pyrites. 



