part 4] DISTRICT ABOUND COBBIS AXD ABEBLLEFENNT. 517 



of view of fossils, and, if graptolites do occur in any abundance, it 

 appears probable that they are confined to thin bands which, so 

 far, 1 have been unable to find. 



The total thickness of the beds is estimated to be nearly 4000 feet, 

 and the dark slaty group at the base about 200 feet. The group is 

 somewhat thinner than might be expected from the area which it 

 covers, owina- to a certain amount of minor folding and rollino-, 

 so that beds on the same horizon may be repeated in any given 

 section. 



(2) Nod Glas. — The monotonous slaty mudstones of the 

 Ceiswyn Beds are succeeded at once by coal-black shales and mud- 

 stones. In some localities they are very highly-cleaved papery 

 shales, in others they possess a tough, blocky mudstone character. 

 These black shales are traversed by numerous joint-planes. They 

 weather to a pronounced rusty colour, and are strongly pyritous. 



It is a well-known band among local quarrymen, because of its 

 highly characteristic lithologieal characters, and because it marks 

 the lower limit of the slate-bearing rocks which are commonly 

 worked in this countrv. It is called by them the Nod Glas 



( = Blue Mark)- 



This black shale-band, which is about 70 feet thick, makes a 

 very characteristic hollow between the harder Ceiswyn Beds and 

 the mottled Abercwmeiddaw Mudstones above. This hollow is 

 well marked throughout the area, and enables one to trace the 

 position of the band, where it is not exposed, without difficulty 

 (see fig. 2, p. 522). 



Because of its comparatively soft character, it is not commonly 

 Avell exposed, except in occasional stream-sections. The beds of 

 such streams flowing over the Nod Glas are often stained a red- 

 dish brown, and large blocks of breccia may be found in the 

 streams, consisting of weathered fragments of the black shale, 

 cemented into a hard mass by ferruginous material. 



The two best exposures in the area are (1) in the small stream 

 immediately west of Ratgoed Chapel, in the Ceiswyn Valley and 

 called locally Nant yr aur, although this name is not recorded on 

 any of the Ordnance Survey maps; (2) on the south-west side of 

 Craig Hengae, where a small unnamed stream cascades down the 

 steep valley-sloDe into the river Llefenni. In both cases the small 

 streams flow for some distance along the outcrop of the Nod Glas, 

 and the band is fairly well exposed. There are also several smaller 

 exposures : for instance, in Nant Cwmeiddaw, north of Corris. 



In several localities there appears to be evidence of movement 

 along this shale-band, resulting in the shattering of the rock, etc. 

 North-west of Corris the outcrop of the overlying Abercwmeiddaw 

 Mudstones is comparatively narrow, and this may be due to strike- 

 faulting along the general line of the Nod Glas. If such a 

 strike-fault docs exist, it may possibly be the continuation of the 

 Aberllefenni strike-fault, which has been proved on the north-east. 



This black shale-band is very fossiliferous, and graptolites are 



