I20 THE OLDER ROCKS AT ST. DAVIDS. 



read of this quartz occurring elsewhere. The microscope shows 

 that it has the incipient sphoeruhtic structure, but not quite so 

 well developed as in that in the other quarry. The spheres of 

 radiating fibres are usually formed round the large dihexahedral 

 quartz crystals which act as a nucleus, and sometimes around a 

 felspar one : in these cases they are not fully formed, but the 

 fibrous border consists of several interfering sectors. This 

 example differs thus from that in the other quarry by the frequency 

 of a large crystal nucleus to the spheroidal structure, and the 

 non-completion of the sphere usually. Microscopically the reck 

 also differs from the large size, and great abundance of the qaartz 

 double pyramids. When the rock is broken with the hammer they 

 drop out, leaving quite sharp impressions of pyramid form. 



The felspar is orthoclase, crystals much decomposed, as might 

 be expected in such old rocks, especially near the surface : strings 

 and fibres of opacite, and particles of calcite and epidote formed in 

 the altered parts. 



Viridite is present, as in the other cases, in scattered flakes and 

 in spots on the decomposing felspar, also occasionally within the 

 quartz 3 it has no depolarising power j it is partly accompanied by 

 epidote particles. The quartz is full of strings of liquid-enclosures 

 crossing in all directions. 



In another quarry below the Church-school towards the 

 cathedral is seen, again, one of the green basic dykes intercalated 

 in the Dimetian. A thin slice shows that it agrees with the other 

 d}'kes above mentioned, in having a doleritic structure. The 

 substance of the felspar is not well preserved, but the form is 

 retained — they are probably all plagioclase. Black irregular and 

 quadrangular grains of magnetite are scattered all over the slide, 

 and from their connection with other products of decomposition 

 in the felspar, are, perhaps, of secondary origin. Quartz is 

 sparsely present but of secondary introduction, probably, being 

 near a little vein, it contains a few acicular microlites. Two little 

 veins of epidote and quartz run through our slice. Viridite is 



