part 1] DISTRICT ABOUND corris and aderllefenni. 521 



There are two bands within these rusty-weathering mudstones 

 which are somewhat harder than the rest of the Red Vein, and 

 they often stand out, making well-marked features : as, for example, 

 where the Red Vein crosses Foel Crochan, north of Aberllefenni 

 (see fig. 2, p. 522). These harder hands are generally mottled, like 

 the Abercwmeiddaw Mudstones which underlie thein. 



The mudstones are called the Red Vein (Y Faen goch) by the 

 local quarrymen, because of their rusty-red weathering. They are 

 well exposed in a number of places, and may be examined in detail 

 at the Abercorris. Aberllefenni, and Ratgoed Quarries, as also in 

 the Ceiswyn stream, etc. 



At the Abercorris Quarries they yield abundantly, about the 

 middle of the group, Orthograptus truncatus var. abbreviatus 

 Elles & Wood and Climacograptus scalar is var. miserabilis E. & W. 

 The same forms may be collected in the Ratgoed Quarries, at 

 approximately the same horizon. 



The Red Vein is also well exposed in the Ceiswyn stream, and 

 here I have collected several specimens of Dicellograptus anceps 

 Nicholson and Climacograptus scalaris var. miserabilis. This 

 band with Dicellograptus anceps is, I believe, at a slightly higher 

 horizon than that which yields Orthograptus truncatus var. ab- 

 breviate in such abundance at the Abercorris Quarries and 

 elsewhere. 



The most abundant graptolite is Orthograptus truncatus var. 

 abbreviates. It is generally very well preserved in full relief, and 

 is extremely abundant in most localities where these beds are 

 exposed. Specimens of Dicellograptus anceps are comparatively 

 rare, and may be confined possibly to a narrow shale-band within 

 the group. This clearly corresponds to the Zone of Dicellograptus 

 anceps. 



(2) Narrow Vein. — The Red Vein is succeeded by a compara- 

 tively thin bed of dark-blue homogeneous mudstone, which is 

 locally called the Narrow Vein (Y Faen gul). This band 

 of mudstone is well cleaved, and constitutes the most important 

 slate-bed in the Corns area. It is practically a pure, clean slate, 

 remarkably free from pyrites, etc. 



The outcrop of this slate-bed is not difficult to follow over the 

 greater part of the area, on account of the numerous quarries which 

 are located on it. Beginning in the south-west, it is exposed almost 

 continuously from the Ty'n-y-berth Quarries, through Ty'n-y- 

 ceunant Quarry into the Braich-goch Quarries, and thence into 

 the Gaewern Quarries, across the Afen Deri to the Abercorris 

 Quarries. From the last-named point, the band trends in a 

 general north-easterly direction across Mynj^dd Abercorris and 

 Grodre Fynydd, until it is again well exposed in the Aberllefenni 

 Quarries on both sides of the Llefenni Valley. Thence it crosses 

 over Foel Crochan, and is exposed once more in the Cymerau 

 and Ratgoed Quarries in the north-eastern part of the area 

 described. 



2n2 



