﻿564 DR. T. GEOOM AND ME. P. LAKE ON THE [NoV. I908, 



south-east of Llechrhydau. Beyond this crag we know of no 

 further exposures. 



Throughout its course, the band consists mainly of a distinctly- 

 clastic rock (see fig. 6, p. 562), which has very generally been greatly 

 sheared so as to assume a schistose appearance, the foliation being 

 oblique to the band. It is usually full of included fragments of 

 various sizes, and these weather out and give to the rock a charac- 

 teristically rough and rugged surface. Among the fragments we 

 have found pieces of slate and rhyolite. 



At some places, however, the appearance of the rock is entirely 

 altered, and it becomes a compact felstone,^ with included crystals 

 of quartz and felspar. This is the case, as already noted, in the 

 crag which overlooks Pandy, in the China-clay Quarry, and in the 

 quarry at Cae Deicws. Between these points the ash is perfectly 

 normal. 



Wherever the felstone occurs the band is greatly thickened, and 

 forms a kind of boss which seems to rise up into the beds above. 

 At one time we were disposed to believe that the felstone was 

 intrusive ; but microscopic sections show that it is really a fine 

 rhyolitic tufi^, consisting of comminuted pumice and exhibiting the 

 characteristic ' bogen-structure' (see fig. 7, p. 562.) 



At Cae Deicws the felstone is very white and contains crystals 

 of chalybite, which are apparently of secondary origin. It shows 

 well-developed columnar jointing (see fig. 8, p. 563), the columns at 

 the eastern corner of the quarry becoming curved, apparently owing 

 to the presence of a small fault. In this quarry there is often a 

 perfectly sharp line between the felstone and the more usual 

 schistose ashes, but this seems to be due to the fact that the 

 boundary between the two at this point is a fault. 



In the China-clay Quarry there is a fine section of nearly the 

 whole of the band. The actual base is cut off by a fault, but the 

 lowest beds that appear are ashy and schistose. In the middle of 

 the quarry the rock is a compact felstone, sometimes white, with big 

 crystals of chalybite, and sometimes blue ; while the upper portion 

 of the band is again distinctly clastic, consisting of a tuff with 

 large fragments. The uppermost layers of this tuff contain broken 

 brachiopod-shells (fig. 9, p. 565). There is a perfect transition from 

 the compact felstone of the middle of the quarry to the coarser tuff 

 on both sides, and no line can be drawn between them. Towards its 

 margin the felstone includes fragments of slate and rhyolite, while 

 towards the interior of the mass these fragments gradually become 

 increasingly difficult to distinguish. 



At Craig-y-Pandy a line of disturbance passes along the back of 

 the crag, and is clearly shown in the small quarry that has been 

 opened there. On the northern side of the disturbance the rock is 

 of the usual ashy character ; but on the southern side, in the boss 

 which forms the crag, it is mostly a compact felstone. Towards 

 the base, however, schistose and ashy beds again appear, and no 



1 We employ the term felstone as a field-name for a rock similar in 

 appearance to a quartz-porphyry, but not necessarily intrusive. 



