part I] DISTRICT AEOU> r D COERIS AXD AEEBLLEFEJfNI. 519 



Abercwmeiddaw Group. 



The Nod Glas is succeeded at once by a group of mudstones 

 possessing very characteristic lithological and faunal features. 

 They consist of pale greyish-blue, thickly bedded mudstones,. 

 rather paler than any of the other Upper Ordovician rocks in 

 this area. The}' are especially characterized by a coarse mottling, 

 consisting of large dark patches which measure as much as an 

 inch in diameter, and are irregular in outline. At some horizons, 

 particularly in the lower part of the group, calcareous nodules- 

 are fairly abundant, and the group as a whole appears to become 

 rather more calcareous as it is traced along its outcrop from 

 south-west to north-east. 



Locally, parts of the group lose the mottled character, and pass 

 into well-cleaved greyish-blue slates. These somewhat irregular 

 slate-bands, which are commonest in the lower part of the group, 

 have been Avorked in several places for slates and slabs : as, for 

 instance, at the Abercwmeiddaw and Cambergi Quarries. They 

 cleave well, and often make good roofing-slates or may be sawn 

 into large slabs. These slates may readily be distinguished from 

 the other slates quarried in the area by their comparatively 

 light grey colour and their texture. The slate in this group is 

 called the Broad Vein (Y Faen lydan) by the local quarry men,, 

 although the slate quarried is not everywhere on exactly the same 

 horizon. 



This group of mudstones, which weathers brown, is well exposed 

 throughout the area, and generally gives rise to somewhat rough, 

 craggy ground. In some places there are large quarries, par- 

 ticularly in the lower part of the group : as, for instance, the 

 Abercwmeiddaw and Cambergi Quarries already mentioned. In 

 addition, there are numerous smaller trial-quarries : for example,. 

 Cwm-dylluan Quany, and on the south side of the Ceiswyn 

 valle} r , etc. 



The Abercwmeiddaw Group contains only shelly fossils, so far 

 as I am aware ; but they are somewhat rare in the Corris-Aber- 

 llefenni area. It is difficult to extract recognizable fossils from 

 the rocks in situ, because the mudstones are tough and split with 

 difficulty along the bedding-planes. In addition, the fossils are 

 scarcely distinguishable on freshly broken pieces, possessing (as- 

 they do) the same general colour as the matrix in which they are 

 preserved. On weathered fragments and in the tips from the 

 quarries in the group, on the other hand, fossils are more readily 

 distinguished, and weather to a conspicuous brown colour. 



There is little doubt that the group, as a whole, is more fossili- 

 ferous in the lower part. At the same time, it should be remem- 

 bered that most of the quarries and trial-workings are situated in 

 the lower part, and it is from these only that I have been able to 

 obtain fossils. The upper part is well exposed in numerous crag- 

 and stream-sections ; but it has not yielded any fossils capable of 

 identification. 



Q. J. G. S. No. 310. 2n 



