124 THE OLDER ROCKS AT ST. DAVIDS. 



It must be confessed, moreover, that we know another sphoeru- 

 litic porphyry of Pre-cambrian, but of Post-pebidian age, the one 

 described by Dr. Hicks {I. c, p. 236) below Nun's Well. The 

 chief points of a microscopic examination are as follows : — 



Sr)h(BrulitiG porphyry near Nun's Well. The spheres are 

 abundant in the ground, but of small size, appearing] as white 

 grains to the unaided eye. They show the cross in polarized 

 light, but the radiating fibration is not so well preserved as in the 

 cases mentioned above. Altogether the rock is more decomposed ; 

 the base contains a large quantity of minute specks of the viriditic 

 matter which has lost any polarising power. It penetrates into all 

 the interspaces. A few cases of viridite pseudomorphing augite is 

 seen, it remains dark under crossed nicols, and is partly covered 

 with opaque black matter, magnetite r 



Felspar. Both orthoclase and plagioclase are present, but much 

 decomposed and full of splashes of viridite. 



Quartz is present both in irregular layers, and larger crystals of 

 regular form, but with rounded outline 3 these enclose masses of 

 viridite, and lines of stone enclosures crossing the interior, or remain- 

 ing roughly parallel to the border ; similar grains are collected on 

 the external outlines, forming a sharp border to the quartz. 



This rock differs considerably in appearance from those 

 mentioned above, having very much less quartz, it seems to have 

 also more plagioclase. The spheres are not formed round large 

 quartz nuclei, but independently in the ground. No epidote was 

 noticed. Altogether it seems a less acidic rock. As it pierces the 

 Pebidian it must be considerably newer than the block enclosed in 

 the basal agglomerate of that series. 



With the Pebidian seems to cease the era of sphoerulitic-porphy- 

 ries in this district j the igneous rocks of succeeding formations 

 are of different order. In the Cambrian near Porth Clais is intruded 

 greenstone with no similarity to any of the older rocks. In the 

 Lingula flags above Porthyrhaw, a felsite of which the same may 

 be said. In the Arenigs of St. David's Head, a coarse-grained 



