﻿370 ME. J. F. I^. GREEN OX THE GrEOLOGICAL [Aug. I908, 



In the Pebidian rocks so far described the ground-mass is largely 

 chloritic even when iron (red)-stained. The higher beds now to be 

 described have a difierent type of ground-mass, but the change is 

 not abrupt, being rather of the nature of a passage. 



(C) The Caerbwdy Series. 



These higher beds cover nearly all the volcanic area east of 

 St. David's. Their junction with the underlying Treginnis Series 

 is clear only in a strip of country, barely 600 yards wide, running 

 east and west through Treginnis-isaf, south-west of St. David's, in 

 Avhich a nearly complete Pebidian succession has been preserved. 



The Caerbwdy Series is always felspathic, and has a characteristic 

 bluish-green coloration, often mixed with white. The texture in 

 hand-specimens changes greatly from one horizon to another, vary- 

 ing from a coarse conglomerate to a halleflinta; under the micro- 

 scope the matrix presents a characteristic appearance throughout 

 the series. It consists of a clear mosaic of quartz, the minute 

 components of which range commonly from *01 to -10 millimetre in 

 diameter ; but it varies iu texture from point to point, coarser and 

 finer patches and ramifications being irregularly intermingled. 

 Chlorite is always intimately intermixed with this ground-mass, and 

 when plentiful aggregates into irregular patches. The mineral is 

 pale in thin sections, fibrous, and of low birefringence. Granular 

 epidote occurs in nests and strings. Embedded in this matrix, 

 throughout the series, are scattered broken crystals of orthoclase 

 and oligoclase, measuring from '2 to "8 millimetre in length. 



This matrix is clearly the result of the alteration o£ a fine-grained 

 acid ash, and in a few cases small concave fragments may be 

 recognized in ordinary light ; but they are indistinguishable between 

 crossed nicols, owing to the silicification which the rock has under- 

 gone. This series attains a thickness of at least 1500 feet, and can 

 "be studied most easily in the magnificent section along the Caerbwdy 

 Valley, about a mile east of the city. The lowest portion of the 

 series (C 1) forms a passage from the basic Treginnis to the acid 

 Caerbwdy rocks. It is only exposed east of Treginnis-isaf, the 

 lower part having a greenish matrix, with red and green enclosures, 

 the upper part being felspathic, green and white. It has not been 

 separated from C 2 on the map (PI. XLIY). 



The next division (C 2) has been termed by previous observers 

 the Clegyr Agglomerate, but should rather be classed as a 

 conglomerate. It is highly felspathic and variable in texture, 

 with bands of porcellanite. The normal white and bluish-green 

 matrix contains patches (often an inch in diameter) of the black 

 or greenish - black chlorite, previously referred to. In this are 

 -embedded rolled pebbles of halleflinta and quartz-porphyry of all 

 sizes, sometimes exceeding 30 centimetres in diameter. 



Above the Clegyr Conglomerate comes a finer-grained rock 

 (C 3) composed of the same materials. !N^o definite line can be 



