51G PEOF. W. J. PUGH ON THE GEOLOGY OF THE [vol. Ixxix, 



capable of identification, for some years ago I saw sample slates 

 from this group, obtained from one of the quarries at the head of 

 the Llefenni Valley, which had fairly well-preserved graptolites on 

 their edges. At that time, however, I had no facilities for exa- 

 mining them in detail, and although subsequently, on various 

 occasions, I have searched the actual quarry from which they were 

 obtained, I have been unable to find the graptolite-bearing band. 



Prof. Cox & Mr. Wells (Eep. Brit. Assoc. 1915,' p. 425) record 

 farther west Amvlexograptus arctus and Qlyptograptus tereti- 

 wsculus var. euglgplius from the lowest beds of their Tal-y-llyn 

 Mudstones, and I have little doubt that these fossils occur on 

 much the same horizon as the slaty beds described above. They 

 state, therefore, that these forms indicate ' .... a high horizon in 

 the Grlenkiln or, in other words, a low horizon in the Caradocian.' 



This dark slaty series is succeeded by the main mass of the 

 Ceiswyn Beds. The cleavage in the higher beds is not generally 

 so conspicuously developed as in the lower, nor are the beds so 

 dark. They are gre} T ish-blue slaty mudstones, with gritty bands 

 up to an inch or more in thickness. Some parts of the group 

 present a banded appearance, oAving to alternation of paler and 

 darker mudstones. Pyrite-cubes occur commonly along the 

 bedding-planes. The greater part of the group possesses a pale 

 yellowish-brown weathering, except the highest beds underlying 

 the Nod Grlas, which commonly show a well-marked rusty-brown 

 weathering. It may be mentioned incidentally that some parts 

 of the Ceiswyn Beds have lithological characters similar to parts 

 of the Upper Valentian or Tarannon rocks of Central Wales. 



It appears impossible to separate this group of mudstones into 

 subdivisions that are capable of being mapped with any degree 

 of accuracy. The slight lithological differences which characterize 

 different parts of the Ceiswyn Beds merge one into the other so 

 insensibly that they afford little or no assistance in surveying. 

 The dark slaty group at the base might be mapped in a general 

 way ; but its upper limit is very indefinite. 



The Ceiswyn Beds are well exposed throughout the area ; but 

 two sections, in particular, may be mentioned as affording good 

 continuous exposures — the Llefenni Valley from Cambergi to the 

 junction with the volcanic rocks, and the line of crags on Ffridd 

 Newydd, on the west side of the Ceiswyn Valley, in Avhich all the 

 beds from the black graptolitic shales of the Nod (Has to nearly 

 the base of the group are well exposed. 



I have several times worked through stream and other sections in 

 this group, but have so far failed to find any fossils, except some 

 imperfectly-preserved graptolites at one point in the Ffridd 

 Newydd Crags. Prom the latter spot, I have obtained badly- 

 preserved specimens, all of which may probably be referred to the 

 species Climacograptus scharenbergi Lapworth. These graptolites 

 occur about 1500 feet below the Nod Glas, in rather dark shales 

 with rusty Aveatherino' tints. 



The Ceiswyn Beds as a whole look unpromising from the point 



