Vol. 6S.'] PKE-CAMBKIAIS^ AND CAMBRIAN OF PEMBROKESHIRE. 383 



finer-grained, and slightly more basic rock is included, in the form 

 •of abundant patches which reach several inches across. 



The main Erimaston mass is exposed in a small quarry about 

 half a mile east of Brimaston, and in the road near Broadway. It 

 presents characters similar to that already described and, like it, 

 includes many basic patches. Along the remainder of the outcrop 

 ■only debris is obsert'able, but this is everywhere abundant. 



At Brimaston, especially in the farmyard of the Hall, the granite 

 appears to pass gradually into the quartz-porphyry described 

 below. 



The Brawdy mass, which is cut up by faults, may be seen at the 

 surface in the farmyard of Brawdy Farm, on both sides of the 

 valley near Silver Hill, and flanking the fault-valley between 

 Troed-y-rhiw and KDaveston. It has been dug for sand by the 

 roadside to the west of Knaveston, near a faulted patch of Lower 

 'Cambrian sandstone. It is somewhat variable in type, but agrees 

 with the Brimaston mass in all essential characteristics. 



(ii) The Quartz-Porphyry. (Dl) 



The quartz-porphyry forming the various masses shown upon the 

 ■map has a slightly variable texture, and exhibits slight variations in 

 colour ; but all these varieties are linked together by the large size 

 and abundance of the quartz-phenocrysts and the relatively fine- 

 grained nature of the matrix in which these phenocrysts are set. 



The main mass of the quartz-porphyry, which occupies the country 

 •around Brimaston and Hayscastle Cross, is a much- weathered rock 

 of a pale yellow to buff colour, having abundant rounded quartz- 

 •crystals which measure up to a quarter of an inch in diameter. 

 The outcrop of this rock is covered with its debris, but exposures in 

 the solid are rare, as the rock has weathered to a great depth. The 

 fiat character of the ground has perhaps prevented in a great 

 measure the natural removal of the weathered rock. 



Shallow pits have been dug in this mass at various localities, as 

 at Barnard's Hill behind the farm, and the hollow 400 yards to the 

 west. A variety with extremely large quartz-crystals also occupies 

 the ground close to Broadway Farm, and rises from the drift- 

 covered moor to the west of Mountain Water. Portions of the 

 rock occasionally show a distinct purplish or pinkish tinge, and 

 contain amethystine quartz ; but in the greater portion the quartz is 

 'either colourless, or opaque-white. 



Small masses of a similar rock occur to the north of Barnard's 

 Hill, near St. Lawrence, at Hayscastle, and at Brawdy ; and a 

 -small elliptical outcrop was detected among the Pebidian purple 

 tuffs on the north side of the hollow, half way between Tre-rhos 

 and Ffynnon-gron. 



(iii) The Diorite. (D\) 



The Knaveston diorite is a fairly compact, dark-grey, fresh rock, 

 «peckled and sometimes stained green. It is of moderate grain. 



