Il8 THE OLDER ROCKS AT ST. DAVIDS. 



porphyry^ which must have flowed over or through it subsequently 

 to the metamorphism of the former. There would be thus a 

 considerable interval between the Dimetian and the porphyry : the 

 latter, perhaps, may be of early Pebidian age. The basic dyke was 

 probably subsequent in date to the porphyry, and may be of late 

 Pebidian age, since that series contains green basic rocks, probably 

 contemporaneous flows intermingled with bedded ashes or ashy 

 shales, and as before stated, they are subsequent to the meta- 

 morphism of the Dimetian. They might of course be long 

 subsequent to the Pebidian, but we have not noticed them in the 

 Cambrian, the intrusive rocks of which are of different character 

 so far as we know. 



A thin slice was prepared of the quartz-porphyry. To the un- 

 aided eye it is a pale greenish-grey rock, weathering slightly reddish 

 from oxidation ; it contains double pyramids of quartz with 

 rounded edges generally ; they drop out of their nidus with facility, 

 leaving impressions of the hexagonal pyramid ; they are surrounded 

 with a white layer having an incipient sphoerulitic structure. The 

 rock in colour is not very different from the Dimetian, but differs 

 plainly even to the eye, as to texture. Under the microscope the 

 appearance is totally different, viz., as follows : — 



Sphoerulitic porphyry ir ova Board-School quarry. The spheres 

 are free, or interfering sectors sometimes formed round quartz 

 nuclei which are obscure dihexahedra, with rounded angles for the 

 most part ; the radiating fibres polarise ; quartz nuclei usually, 

 when present, cause those spheres to be larger than the others. 



A mineral in diverging irregular prismatic form, pale greenish- 

 yellow in colour, but polarizing a vivid green and crimson, I take 

 to be epidote ; it is abundant — apparently not dichroic in this case : 

 we meet with this product of rearranged constituents in other 

 cases as will be noticed. Viridite is also present and in some cases 

 has retained the form of augite prisms 5 but is converted into a 

 fibrous state, and has lost the polarising power. A few acicular 

 prisms, as of apatite, are present. 



