Yol. 68.] PEE-CAMBEIAN AND CAMBRIAN OF PEMBROKESHIRE. 385 



seemingly been furnished by quartz-keratophyres, and pebbles of 

 felspathic quartzose sedimentary rocks. 



The matrix of the Pont-yr-hafod rocks is extremely fine-grained, 

 and under the microscope has almost a glassy aspect, owing to the 

 little action that it has on polarized light. It appears to be highly 

 siliceous, and is slightly iron-stained. In the finer tuft's the matrix 

 is not so siliceous, neither does it consist of so finely comminuted 

 material. 



One other mass referable to this group of rocks occurs as a 

 faulted lenticle, and is presumably surrounded by younger beds. 

 It forms a striking mound in the bottom of the fault-valley which 

 ranges from Gignog to Knaveston. The rocks are extremely rotten, 

 and have lost most of their original structure ; but it is possible to 

 make out (E 7244, 7245) that the fragments were of andesitic 

 character, and consisted of plagioclase felspar- crystals set in a fine- 

 grained and vesicular ground-mass. The whole rock now consists 

 of a mass of chlorite, secondary quartz, and iron-ores. The felspar- 

 phenocrysts are replaced by a mosaic of secondary quartz, and thua 

 stand out in contrast with the matrix, which is chloritic and iron- 

 stained. Considering the mass as a whole, it probably was slightly 

 more basic in composition than most of the tuffs of the Pont-yr- 

 hafod Group exposed around Pont-yr-hafod. 



The Ehindaston and Gignog Group. — Unlike the rocks of 

 the Pont-yr-hafod Group, the acid group of Rhindaston and Gignog 

 had been detected in places by previous observers, who have from 

 time to time furnished descriptions of certain of its types. 



The majority of the rocks of this group may be classed as- 

 quartz-bearing soda-rhyolites or quartz-keratophyres^ 

 and their attendant tuft's. 



Specimens for examination have been collected from a variety of 

 localities, but chiefly from the good exposures near Gignog, from, 

 the crags on the east side of Brandy Brook, and from the cliffs- 

 between Pointz Castle and Perth Mynawyd. The Gignog rocks are 

 typical of the group. A specimen collected from the western side 

 of the Brandy Brook valley, to the east of Asheston, proved to- 

 be a quartz-keratophyre (8992), and is shown in fig. 1 A (p. 386). 

 It consists of corroded quartz-phenocrysts, measuring some few 

 millimetres across, and well-shaped phenocrysts of albite, set in a 

 fine-grained devitrified and silicified ground-mass in which the 

 original fluxion-structure can be distinctly made out. 



The associated tuffs are composed of exactly similar materials, 

 and only differ from the above in their obviously fragmental 

 character (E 7246, 7258). Usually the larger fragments in these 

 tuffs are not rock-fragments, but broken crystals. Occasionally, 

 however, the tuffs become coarser and well-banded (E 7247, 7248) 

 with the introduction of more finely clastic material in the form of 

 fine-grained sedimentary rocks of quartzose and argillaceous, 

 character, while chloritized fragments of andesitic rocks may be 

 present in some quantity. 



