THK OLDER ROCKS AT ST. DAVIDS. I23 



Quartz grains, mostly of irregular form, are scattered among 

 them in the microfelsitic ground ; occupying spaces between 

 the spheres. Some dihexahedral crystals are present, containing 

 inclusions of ground matter, microlites and minute, perhaps liquid 

 cavities, though a moving bubble could not be detected Much 

 viridite is present between the spheres, scattered in spots and grains 

 throughout, and in marked linear arrangement ; itself the product 

 of decomposition, it is frequently encroached on by brilliantly 

 polarising patches which seem to be epidote. Brilliant groups 

 of epidote prismatic crystals are found, apparently of secondary 

 origin, and formed subsequently to the viridite constituent. In 

 some cases the epidote group occupies the centre of the decomposing 

 felspar crystal. Scattered felspar crystals are orthoclase showing 

 the Carlsbad twinning 5 one case of obscure banding would show 

 a plagioclase crystal. 



Though this does not precisely correspond as far as the fluidal 

 disposition is concerned, the mineral composition is identical 5 

 hence, one may be a portion of a current which solidified nearer 

 the surface, developing fluidal lines and perfect sphcerulites, that 

 seen in situ in the quarry may be the deeper part of a dyke, or 

 have consolidated further below the surface, or belong to a 

 slightly different position of the same eruptive period, as a lava of 

 Monte Nuovo may differ from one at Vesuvius a few miles off, 

 and a century or so apart. 



If this be allowed, we have, 1 think, presumptive proof of the 

 age of the quartz-porphyries N. of the Cathedral ^ they would be 

 about the age of the lowest beds of the Pebidian. Hence, if they 

 have produced contact-alteration of the Dimetian rocks, baking, 

 hardening them, and altering their appearance as above described, 

 we have here corroborative evidence of a lapse of time between the 

 Dimetian and Pebidian periods. This argument supplementary of 

 those advanced by Dr. Hicks, is put forward with hesitation as 

 the result of only a short investigation on the spot. Not that any 

 further corroboration is necessary, for we may take, I think, the 

 result of Dr. Hicks' researches as well founded. 



