﻿368 ME. J. F. N. GHEEN ON THE GEOLOGICAL [A.Ug, 1908, 



Eamsey Sound, it covers a wide area and is coarser than elsewhere. 

 In hand-specimens, the rock is white or pale green, and contains 

 scattered, subangular, pink or purple fragments ; these sometimes 

 weather out, leaving ferruginous hollows. Under the microscope, 

 the rock is seen to consist essentially of grains of quartz and 

 decayed felspar, set in a dusty epidotic matrix, Avith strings of 

 chlorite and occasional fragments of felsite. The characteristic 

 purple fragments are apparently' hornblende-trachyte containing 

 numerous vesicles. The ground-mass consists of small felspar- 

 laths, extinguishing straight, showing marked fluidal arrangement, 

 mingled with a great number of minute ferruginous grains to 

 which the colour of these fragments is due. The vesicles are filled 

 with some mineral, tending to form radial aggregates, which may 

 be a zeolite. A few of the phenocrysts are orthoclase, but the 

 majority are ferruginous pseudomorphs after hornblende, which in 

 some cases contain corrosion-channels. The nature of the ground- 

 mass of the tuff, in which these purple fragments are embedded, 

 shows clearly that the ferruginous infiltration or replacement 

 occurred before deposition in the tuff. 



Not more than 150 feet of A 1 appears to be shown, and the 

 remainder of the Penrhiw Series is about 800 feet thick, most of 

 which is formed of A 2. 



Schistose sill. — Above A 3 there occurs almost invariably a 

 foliated quartz-felspar-porphyry sill, to be described later. 



(B) The Treginnis Series. 



The tuffs of the second or B series cover a large area to the 

 west of St. David's, and are specially well shown on the farm of 

 Treginnis-uchaf. The series is distinguished from all others by its 

 basic character and by the occurrence of scattered fragments, fairly 

 well rolled, varying from 2 to 20 centimetres in diameter, of red 

 rock, described as a quartz- andesite,^ which seems, however, to be a 

 rhyolite. 



The lowest bed of the series (B 1) is a gritty tuff composed of 

 dull-red fragments in a green base. There is considerable variation 

 in the nature of these fragments, the most abundant being a red, 

 vesicular, glassy trachyte, very similar to that which will be pre- 

 sently described, but with much ferruginous matter. Less abundant 

 are fragments of red glass with augite- and plagioclase-phenocrysts, 

 a red rhyolite, and a rock apparently identical with the hornblende- 

 trachyte of A 3. The thickness of B 1 is very variable, from a mere 

 passage-bed a few yards thick at Penyfoel to at least 150 feet near 

 llhoson. It passes gradually into B 2, and has not always been 

 separated from it on the map (PI. XLIV). 



1 C. Lloyd Morgan, 'The Pebidian Volcanic Series of St. David's' Quart. 

 Journ. Geol. Soc. vol. xlvi (1890) p. 258. The exposures marked on his map 

 as diabase-tnfF with quartz-andesite belong to this series. 



