﻿Vol. 64.] BALA AKD LLANDOVEKY EOCKS OF GLTN CEIRIOG. 569 



again visible until we reach the immediate neighbourhood of the 

 Glyn Valley, where it is well shown in Coed-y-glyn Quarry. On 

 the other side of the valley it is exposed only in a small trial- 

 working some distance above the China-clay Quarry; here it is 

 stated to have thinned out as working proceeded. Farther east, 

 the only point at which it has been definitely identified is in 

 Ty-nant Wood, close to the cottage. 



By far the best exposure is that exhibited in the quarrj^ at 

 Coed-y-glyn. The rock is compact and generally blue in colour, 

 at least in its central portions. Towards the top and the bottom 

 it is often vesicular. Both the lower and the upper surfaces are 

 very irregular, and the slates close by are sometimes indurated, so 

 that the sheet seems to be distinctly intrusive. It is probable, 

 however, that some of the irregularity is due rather to earth- 

 movements than to the intrusive nature of the rock, for the beds 

 at its base are much disturbed ; but, how far these movements may- 

 be the effect of the intrusion, we are unable to say. The rock in 

 places shows columnar jointing. 



(5) Pen-y-graig Ash. — The intrusive sheet that has just 

 been described is the rock which has commonly been taken for the 

 ' Little Ash ' of Jukes, and it is possible that the Geological Survey 

 itself has sometimes mapped it as such. But a glance at the Survey- 

 map will show that the line of the 'Little Ash' in the Glyn 

 Valley is drawn, not through Coed-y-glyn Quarry, but considerably 

 higher in the series, through the farm of Pen-y-graig. Here there 

 is a true ash-bed, of which, however, only a very small portion 

 exists at the surface, the rest being cut out by the strike-fault. 

 This Pen-y-graig Ash is much more like the * Little Ash,' as this 

 is seen at Llandrillo and elsewhere. A small outcrop of similar 

 rock, not more than 4 or 5 feet thick, is exposed in the quarry at 

 Bryn, lying immediately above the sandstone mentioned on p. 566. 

 Finally, in the valley of Craignant a band of similar ash, not more 

 than 2 or 3 feet thick, may be traced at intervals running nearly 

 along the line of the stream in the upper part of the valley, where 

 it overlies a sandstone-bed. It is, however, by no means certain 

 that all these exposures belong to the same band, and the great 

 distance of the Craignant ash from the nearest outcrop of the 

 Craig-y-Pandy band suggests that the former belongs to a much 

 higher horizon. 



(6) Graptolitic Slates. — Over the greater part of the area 

 shown upon the map, the strike-fault that cuts off the top of the 

 Pandy Beds is followed directly by the Dolhir Series, as in the 

 Ceiriog Valley (fig. 1, p. 552). In the west of the area, however, 

 the Dolhir Beds are underlain by dark graptolitic slates, the 

 graptolites in which are unfortunately very badly preserved. 

 Owing to the fact that the Dolhir Fault cuts obliquely across 

 the strike, the greatest thickness of these slates is to be seen in 

 Nant Tyn-y-twmpath in the extreme west of our map (PI. LlII) 



