PRE-CAMBRIAN ROCKS OF ST. DAVID'S. 315 



ridge into microcrystalline and spherulitic porphyry. The rock 

 seen at Rock House is granitic in texture, and consists of the same 

 minerals as the adjacent granite of Bryn-y-garn, but in smaller 

 forms. The quartz and felspar, in a kind of granular micropegma- 

 titic intermixture, constitute nearly the whole mass ; but the chlo- 

 rite is also present in tufted bright green aggregates. The rock on 

 the roadside is still more finely crystalline. It forms a stage 

 between granite and true felsite. Its ground-mass presents a 

 microcrystalline aggregate of quartz and felspar, similar to that of 

 the Church-School quarries, through which also spherulites are 

 distributed. The same chloritic constituent so characteristic of the 

 granite, is still recognizable here. Short of an actual section show- 

 ing the gradations of the one rock into the other, I do not think 

 that better evidence could be found that the granite is directly 

 connected with the porphyries that lie along its border. 



This intimate relationship is further illustrated by a study of the 

 minute structure of the porphyries. Some of these rocks nave been 

 described in Dr. Hicks's papers by Professor Judd and Mr. Davies, 

 and elsewhere by Mr. Tawney. Mr. Davies has shown the thoroughly 

 crystalline nature of the ground-mass of the rock at the Church-School 

 quarries. My observations not only confirm his description, but ex- 

 tend it to all the porphyry masses of the district. In none of them have 

 I noticed any true felsitic base, though this may yet be found. Their 

 ground-mass, between the spherulites to be immediately referred to, is 

 entirely microcrystalline, and is resolvable into a granular inter- 

 mixture of quartz and orthoclase. The rocks are quartz porphyries, 

 but with a remarkable development of spherulites, which, following 

 Vogelsang's terminology, are felso- spherulites. This spherulitic struc- 

 ture has been developed in altogether exceptional perfection and beauty 

 (Plate X. fig. 9) — so much so, indeed, that the spherulitic quartz por- 

 phyries of St. David's are no doubt destined to become as classical 

 examples of this structure, and as much sought after for collections 

 of microscopic petrography, as the pitchstone of Arran now is for 

 its microliths. They have been well described by Mr. Davies as 

 they occur in the Church-School quarries*. He has pointed out the 

 want of peripheral definition of the spherulites in the rock at that 

 locality, the absence in them of a central nucleus, and their ten- 

 dency to group themselves round the quartz and felspar crystals. 

 Instances, however, of sharp borders to the spherulites may be found 

 in this rock, and still more conspicuously in the mass exposed at the 

 centre of the cove below Nun's Chapel. The rock at the latter 

 locality is finely spherulitic, the spherulites having a distinct dark 

 border, and many of them standing isolated in the base. Though the 

 base is somewhat decomposed, the spherulites are still tolerably fresh, 

 and react in the usual way on polarized light, giving a distinct black 

 cross between crossed prisms (Plate X. fig. 10). 



The development of these spherulites is one of the problems which 

 will require a more exhaustive study of the St. David's rocks, and 

 the solution of which can hardly fail to throw light on the relations 

 * Quart. Journ. Geol. Soc. vol. xxxiv. p. 1G4, xxxv. p. 293. 



