Vol. 6S.'] PRE-CAMBRIAN AND CAMBRIAN OF PEMBROKESHIRE. 387 



The rocks of the Pointz-Castle mass are those which in the past 

 were classed by Hicks as part of his Arvonian. A petrographical 

 description of a rock of this group was given by Dr. Bonney, 

 who compared it with the rhyolitic ash of Clegyr Bridge in the 

 St. David's district. The bulk of the Pointz-Castle mass consists 

 of fragmental material, either in the form of breccia or in that of 

 ash ; but quartz-keratophyres occur, and, as pointed out by Hicks, 

 some of them appear to have been vesicular (E 7226). 



In a rock collected from the upper end of Forth Mynawyd near 

 the lime-kiln, the quartz-phenocrysts reach a fair size (E 7252), and 

 are much more abundant than the albite-phenocrysts. The ground- 

 mass is a microcrystalline mass of quartz and albite, with a little 

 sericitic material. 



One of the vesicular rocks in which the vesicles are filled with a 

 mosaic of secondary quartz occurs at the base of the cliff on the- 

 northern side of Cwm Bach, Newgale. This rock has also a 

 tendency to show spherulitic structure in the ground-mass. 



(ii) The Dimetian. 



The granite. — The normal granite of Brawdy and Brimaston 

 is a fairly fine-grained, yellowish-grey rock, which contains a variable 

 amount of quartz, felspars, and ferromagnesian minerals. There- 

 is a complete absence of porphyritic structure ; but, occasionally, 

 the quartz may show a tendency towards idiomorphism. The 

 presence of mica was recorded by De la Beche in 1846 (5, p. 230). 



Under the microscope the rock of Silver Hill and Knaveston 

 (E 7249, 7251) seems to be of uniformly medium grain, and consists- 

 of quartz, felspar, and a green pleochroic mineral pseudomorphous 

 after biotite. The most interesting feature of this rock is the 

 nature of the felspars, and their relation one to the other and to the 

 quartz. 



The greater portion of the felspar is much-twinned albite, the 

 crystals of which are often surrounded by perthite, and are in 

 optical continuity with the albite of the perthite. The relative 

 proportions of perthite and albite are variable in different parts of 

 the mass, specimens from Knaveston containing a greater proportion 

 of albite than those from Silver Hill. 



The quartz of much of the rock shows the usual granitic 

 characters ; but occasionally it has a tendency to form pegmatitic 

 or granophyric intergrowths with the alkaline felspars — a tendency 

 which becomes more marked as the texture of the rock becomes 

 less coarse, as at the extreme end of the mass to the east of 

 Brimaston (E 6572). 



Kone of the specimens examined contained microcline, and 

 accessory minerals such as primary sphene, apatite, or iron-ores 

 are not abundant. 



Compared with the Dimetian granite of St. David's there are a 

 great many points of similarity, especially in the structure and the 

 composition of the felspars, for the St. David's rock consists chiefly 



