﻿624 ME. W. G. FEAENSIDES ON THE GEOLOGY OF [Aug. I905, 



The great bulk of both the Platy Ashes and the Agglomerate 

 consists of shattered blocks and particles of a hypersthene-andesite 

 exactly similar to that of the intrusions to be described hereafter. 

 The blocks of the Agglomerate are set in a matrix of smaller 

 particles of the same material, and the whole mass is usually to a 

 considerable extent secondarily silicified (4649-51). Most of the 

 blocks in the Agglomerate are about the size of a hen's egg or of 

 a cricket-ball, but sometimes quite large masses are seen. With 

 these there are also, especially in the northern district, various 

 large and small masses of crushed and hardened, but uncleared, 

 shale, and in one of these several nice specimens of Lingulella 

 Davisii were found. They came from a shale-mass several feet 

 long, which occurred in the .Agglomerate about 100 yards to the 

 west of Rhyd-y-fen. Small pieces of a coarse granophyre, with 

 patches of centrically-arranged quartz-orthoclase eutectic, and 

 idiomorphic oligoclase, have been found in the lower part of the 

 Agglomerate, and are interesting as being unlike any of the other 

 rocks of the district (4652). 



Masses of some ophitic rock, rather like the later dolerites, occur 

 in the higher part of the series. The whole of the Agglomerate is 

 exceedingly massive, and might well belong to a single phase of 

 deposition. It is much thicker in the north and east than in the 

 south, and particularly the south-west, where, though traceable on 

 Moel Llyfnant, it appears to be rapidly dying out. Upwards the 

 Agglomerate passes into the Upper Platy Ashes, which are similar 

 in every respect to those below and equally uninteresting. The 

 thickness of the Agglomerate about Clogwyn Maen Grugog must 

 be something between 300 and 400 feet ; that of the Upper Platy 

 Ashes may be anything up to about 150 feet, and seems to be 

 greatest where the Agglomerate is thinning, as to the east of Craig- 

 y-Bychan. 



The Daerfawr Shales [6] . 



Upwards the Upper Platy Ashes become continuously finer, until 

 they merge into the peculiar series of pyroclastic mudstones to which 

 I have applied Mr. Williams's name, the Daerfawr Shales. 

 These are a most variable series of deposits, and are more in- 

 constant in thickness than even the Basal Grit. Though chemically 

 almost pure eruptive rocks they are often beautifully laminated, and 

 seem admirably adapted for the preservation of fossils. Where thin, 

 they are often quite well-cleaved, and have been prospected for slates 

 in numerous localities. Where thick, they appear uncleaved, and 

 include certain sandy beds, with a certain amount of angular chips 

 of detrital quartz and a few mica-plates. In such beds worm- and 

 crustacean-tracks are quite abundant, and it was from these that 

 Prof. Hughes obtained the problematical Saccocaris major and Sacco- 

 caris minor. Some of the beds show very fine examples of infiltered 

 iron or other colour-staining, which seems to be later than the inci- 

 pient cleavage and jointing, through the cracks of which it has often 



