384 ME. H. H. THOMAS AND PEOr. 0. T. JONES ON THE [Sept. 1912^ 



and has been quarried for local building-stone at a point 400 

 yards to the north-west of Knaveston Cottage. This rock occupies 

 quite a small area, and only one outcrop has been detected. It 

 differs in character from the other basic intrusions which cut the 

 granitic rocks of the surrounding country, and has no parallel in 

 the St. David's region. Dioritic rocks, however, occur among the 

 supposed pre-Cambrian of South Pembrokeshire, as at Johnston,. 

 Talbenny, etc. 



(iv) The Basic Dykes. 



A certain number of basic dykes, which we are inclined to regard 

 as a phase of the Diraetian, cut the Pebidian and also the older 

 members of the Dimetian. Such dykes occur in the granite of 

 Brawdy, where one is seen in the farmyard cutting the granite 

 and abutting against the Cambrian conglomerate ; another cuts the 

 granite in the road near Brawdy Yicarage, and yet another in the 

 granite has been cut into by a small quarry in the valley south of 

 Silver Hill. 



Another basic dyke, measuring about 45 feet across its outcrop, 

 cuts the older Pebidian, and is exposed in the road at Pont-yr-hafod, 

 east of Hayscastle. It is similar in all respects to those mentioned 

 above. These dykes are certainly numerous ; but they are not 

 conspicuous, on account of the covered nature of the ground. 



Generally speaking, they are fairly dark-grey to dark bluish-green 

 rocks of moderately fine texture. 



(c) Petrography. 



(i) The Pebidian. 



The Pont-yr-hafod Group. — Specimens for microscopic 

 examination were taken from the lowest exposed purple-and-green 

 tuffs at Pont-yr-hafod : the best specimens being collected from a 

 small pit behind the schoolhouse (E 8993, 8994).' 



These tuffs are composed of somewhat angular fragments, which 

 represent a variety of rocks of volcanic and sedimentary character,, 

 but no rocks of a plutonic nature have been met with. The larger 

 fragments consist of fine-grained quartz-albite rocks which may be- 

 classed with the keratophyres, but much of their original structure 

 has been destroyed. Other fragments are of the nature of quartzose 

 keratophyric tuffs, and are made up of broken and angular quartz- 

 and albite-crystals in a fine sericitic matrix. Fragments of what 

 were presumably andesitic or basaltic rocks are represented by 

 chloritized and iron-stained masses, in which occasionally a microlitio 

 structure is still observable. 



Among the smaller fragments of the above-mentioned materials 

 occur abundant grains and broken crystals of quartz, which have 



^ These numerals refer to the numbers of the slides in the collection of 

 the Geological Survey. 



