PKE-CAMBETAN ROCKS OP ST. DAVId's. 311 



shales. Where it occurs, the rock is usually pale apple-green to 

 pearl-grey, but occasionally of a pinkish tint, with a silky lustre, 

 soapy feel, and finely schistose texture. These characters have 

 been developed here and there along particular lines or in certain 

 bands of rock, the beds above and below exhibiting no appreciable 

 trace of them. A specimen taken from one of the foliated beds 

 might be supposed to have come from the sericite schists, or fine 

 hydro-mica schists of a district in which regional metamorphism 

 has been well and widely developed. Yet the beds immediately 

 above and below are ordinary tuffs, shales, or sandstones. 



I have had thin slices prepared from specimens collected from 

 different horizons, to show various stages in the development of this 

 foliation (PL IX. figs. 5 & 6). Some of these are from the section 

 on the road-side north from the St. David's Schools ; others from 

 the schists at Porth-lisky and from the same group at Ogfeydd-duon 

 on Ramsey Sound. Others, illustrating the alteration of the beds 

 above the conglomerate, were taken from schistose bands among the 

 ordinary strata at Porth-seli ; at the Life-boat House, Porth Stinian ; 

 and at Porth Cadnaw, a little south from St. John's Point. I can offer, 

 at present, only the general results of a first study of these slides ; 

 but the examination has convinced me that the district is one from 

 which a more exhaustive research could not fail to derive much fresh 

 insight into the early stages of regional metamorphism. 



The original clastic character of the rocks is still everywhere 

 raceable, but is less distinct among the lower and older portions of 

 the series. This probably arises from the original mineral constitution 

 and state of aggregation of the fine fragmentary materials, rather 

 than from relative age and depth. The base of the schists appears, 

 under the microscope, as a felted aggregate mainly composed of 

 minute scales of a nearly colourless mica. These scales are grouped 

 linearly along the planes of foliation, which coincide in general direc- 

 tion with those of bedding. They wrap round the clastic granules, 

 and are not unfrequently interlaced into short folia. The mineral 

 thus developed in the process of foliation is doubtless one of the 

 hydrous micas, so frequently observable in the metamorphic schists ; 

 it may be sericite. Next in abundance to it is an opaque granular 

 substance with no recognizable crystalline form, which has, as it 

 were, been pushed aside by the crystallization of the mica, and is 

 disposed in lenticular and coalescent folia along the general planes 

 of foliation. It is the presence of these sharply defined streaks of 

 black dust which gives so much precision to the line of foliation 

 in many of the slides. Another mineral of secondary or metamor- 

 phic origin is bright green, fibrous, and granular, tufted or vermi- 

 cular. Prom these characters and its behaviour in polarized light, 

 I have little doubt that it is chlorite. It occurs in oval or eye- 

 like nests, but occasionally is prolonged into folia, and sometimes 

 takes the place of the nearly colourless mica. Its fibres are usually 

 disposed transversely to the longer axis of the aggregates. In one 

 slide (Ogfeydd-duon) abundant crystals of pyrites have been deve- ' 

 loped along some of the folia ; in another (Life-boat House), minute 



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