part 4-] gkolociv of cotsrts axd aberllefestni. 515 



instance, at the head of the Llefenni Valley (north of Waun 

 Llefenni), along Nant y Bengiog on Craig y Llani, and in several 

 places on the north-western slopes of Mynydd Ceiswyn. 



At the head of the Llefenni Valley is a continuous section in 

 the stream, from the volcanic rocks into the Ceiswyn Beds. There 

 the highest volcanic rocks consist of well-bedded rhyolitic ashes 

 which are overlain by dark-blue slat} 7 mudstones. There are one 

 or two thin bands of ash} 7 mudstone a few feet above the main 

 mass of the volcanic rocks : but these do not seem to be present 

 in all sections which have been examined. Near the junction a 

 good deal of vein-quartz occurs commonby, and in the Llefenni 

 section this appears to contain traces of lead. Vein-quartz con- 

 taining lead is known to occur at this horizon in several other 

 localities outside the area described in this paper, as, for example, 

 north and north-west of Dinas Mawddwy. 



It may be mentioned that the Mynydd y Waun outcrop lies on 

 the axis of a well-marked anticlinal structure which passes west of 

 Corns, and has been traced for some distance south of that village 

 into the Machynlleth area. 



Bala. 

 Hengae Group. 



(1) Ceiswyn Beds. — Immediateby overlying the highest Ordo- 

 vician volcanic rocks is a great group of monotonous slaty mud- 

 stones, generalty dark blue and greyish blue. It is a thick group 

 which occupies a considerable portion of the area described in this 

 paper. 



The lower part consists of very dark-blue, highly cleaved slates. 

 These dark slates are not homogeneous in character, but are tra- 

 versed by fine-grained gritty bands up to an inch or more in 

 thickness. They have at various times been quarried for roofing- 

 slates, but none of the quarries are at present working: for in- 

 stance, the old quarries at the head of the Llefenni Valley and on 

 Mynydd y Waun. 



These dark slates possess characteristic physical properties which 

 distinguish them from all other slates in the area. They are 

 highly cleaved, and split readily into thin sheets. Fresh cleavage- 

 planes possess a silky lustre. Their economic value is somewhat 

 diminished by the presence of numerous small joints, the thin 

 gritty bands mentioned above, pyrite-cubes which are often 

 numerous along the bedding-planes, and the presence in some 

 localities of a characteristic wavy cleavage. 



I have searched these slates on several occasions for fossils ; but. 

 so far, they have only yielded very imperfectly preserved traces of 

 graptolites. One of these may possibly be Olyptograptus tereti* 

 usculus ; the others are of the Diplograptid type, and are not 

 capable of identification. The slates are so highly cleaved that 

 they are difficult to split along the bedding-planes. However, 

 there is little doubt that they do contain graptolites and also iorms 



