THE OLDER ROCKS AT ST. DAVIDS. 1 19 



The felspar is much attacked, and contains particles of the green 

 brightly polarising epidote constituent. The twins are of the 

 orthoclase type. No plagioclase noticed in our slice. 



Quartz is abundant both as irregular shapes, or dihexahedral 

 crystals producing rhombic, or hexagonal outlines : it contains lines 

 and scattered grains of liquid-enclosures ; sometimes a process of 

 the ground will protrude into the crystal some way. 



Similar rocks are seen in a quarry about loo yards distant, close 

 to the Church-school. Here is the fine-grained Dimetian again 

 i t is very tough, but contains cavities, and seems to me to have been 

 altered by contact with the porphyry j it is finely granular instead 

 of being coarsely crystalline as is its wont. The mineral con- 

 stituents remain, however, the same. This fine-grained rock has 

 a micro-granitic structure : a finely granular crystalline ground 

 with scattered larger felspar crystals 3 there is a somewhat peculiar 

 jagged interlacing, almost obscurely radiate arrangement of the 

 ground : this may be probably due to contact with the 

 porphyry adjacent. On the other hand it may possibly be an 

 intrusive felsite • but the quartz is not dihexahedral. 



The felspar crystals are orthoclase much decomposed, a little 

 plagioclase being present also. 



Viridite is present, and is covered sometimes with red spots as 

 of ferric oxide ; also grains of magnetite are scattered throughout. 

 The epidote constituent also in small quantities, and as in the 

 other cases seems of secondary origin. 



Quartz is in irregular angular shapes, not of large size — no 

 separate regular formed crystals present ; it contains fluid 

 enclosures, and portions of the ground. 



The top of this quarry is occupied by a porphyry similar to the 

 last-named, except that it is much coarser grained. The dihex- 

 ahedral quartz are very large crystals ; the colour and aspect to the 

 eye is much that of ordinary Dimetian, owing to its coarse 

 crystallization 3 it is distinguished, however, by the character of 

 the quartz. On the spot, one classed it as intrusive, as I have not 



