PEE-CAMBEIAN EOCKS OP ST. DAVID's. 313 



examination suggests, indeed, that all the schists were originally 

 tuffs. The green and grey shales lying below and above the con- 

 glomerate, which include such excellent examples of fine silky schists, 

 were not improbably derived in large measure from fine volcanic 

 sediment. 



My second visit to St. David's was especially intended to obtain 

 further data regarding this question. But I am not yet able to 

 throw much light upon it. There can, I think, be no doubt that, 

 in so far as the production of a true foliated structure depended 

 upon the operation of influences entirely outside of the rocks them- 

 selves, closely adjacent strata must have been under practically the 

 same conditions. The pressure, tension, and temperature can 

 hardly have sensibly differed in contiguous rocks. If, therefore, all 

 the rocks were subjected to the same processes, any resulting differ- 

 ences in their present aspect and structure must, I should imagine, 

 be due to some original variety in the chemical composition and 

 physical structure of the rocks themselves. Certain layers or 

 particular kinds of fine detritus, more especially some of the finely 

 comminuted volcanic dust, have been specially susceptible of change 

 along the planes of deposit; and sericite, chlorite, pyrites, and mag- 

 netite have been developed along those planes, so as to produce a 

 marked foliation. 



In the St. David's district we seem to stand in presence of some 

 of the initial stages of that still mysterious process by which wide 

 regions of sedimentary strata have been changed into crystalline 

 schists. 



4. The Geanite, Qjtaetz Poephyey, and accompanying 

 Metamoephism. 



In the first part of this paper sufficient evidence has been adduced 

 to show that at St. David's a central boss of eruptive granite, with 

 associated peripheral dykes, elvans, or amorphous intrusions of 

 quartz porphry, has been protruded through the Cambrian strata. 

 I purpose now to supplement that evidence by discussing more fully 

 the structure and relations of the eruptive rocks, and the influence 

 they have exerted upon the stratified formations through which 

 they have arisen. 



The granite, as has been already shown, lies on the eastern limb 

 of the isocline, where it invades the various rock groups up to the 

 zone of green shales and sandstones that lies some way above the 

 quartz conglomerate. The porphyries are grouped round the cen- 

 tral boss of granite, and appear to be intimately connected with it, 

 like the elvans of granite districts. I shall first describe the petro- 

 graphical characters of these rocks, and then give some account of 

 the metamorphism associated with them. 



Of the granite I have had a good series of thin slices prepared 

 from characteristic specimens taken from all parts of the district, 

 and have subjected them to microscopic examination (Plate X. fig. 

 11). To the descriptions already given by Prof. Bonney, Mr. Davies, 



