Vol. 65.] SUCCESSION AROUND rLYNLBION AND PONT ERWTD. 515 



quarters of a mile this track lies approximately along the strike, 

 and not much rock is visible. Near Craig-y-Cauleth laminated 

 gritty bands are present among the mudstones, but they disappear 

 immediately to the east of that. By the side of the track, about 

 one-third of a mile north-east of Craig-y-Cauleth, there is a small 

 exposure of blue mudstone with rottenstones, some of which are 

 crammed with crinoid-ossicles and minute brachiopods (chiefly 

 Orthis). The only graptolite that I obtained was Monograptus 

 intermedins. In slightly lower beds at the foot of Coed-y-Cauleth 

 (F. 56) I found several species of graptolites : — Monograptus 

 •crassus, M, marri or becJci, M. nudus(?), and M. resurgens. This 

 assemblage indicates, as compared with the Myherin Valley, a 

 horizon near the base of, or slightly below, the blue mudstone- 

 group, which agrees with the position independently assigned to it 

 from stratigraphical evidence. 



This group occupies a width across the strike of about 1000 yards, 

 and is followed eastwards by the group of dark-blue mudstones and 

 shales, the former weathering in their characteristic manner. These 

 rocks are well exposed in the magnificent precipice of Oaig-y-Ceffyl, 

 and graptolites may be obtained from the shaly bands by the side 

 of the cart-road which skirts the edge of the precipice (F. 57). The 

 following species were collected there : — Monograptus priodon(! ), 

 M. turriculatus, Petalograptus palmeus var. tenuis (c), and P. altis- 

 simus (?). The outcrop occupies a width of 400 or 500 yards, but 

 the rocks are probably folded. 



East of Craig-y-Ceffyl the mudstones are succeeded by a shaly 

 group, and in a short distance a bed of coarse speckly grit, resting 

 upon blue shales with greyish-white siliceous bands, may be 

 observed by the side of the cart-track, and is exposed again at 

 frequent intervals along the track for about 300 yards ; this is due 

 to a series of sharp folds, which cause the outcrop to zigzag rapidly, 

 its general trend being nearly at right angles to the axes of the 

 folds (see Map, PI. XXIV). At the end of this space the dip 

 becomes steady towards the east, and the grit strikes northwards 

 without much interruption to Nant Ehuddnant. 



This sharply-folded belt is probably the one so clearly exhibited 

 in the dark-blue mudstones near the head of the ravine of the 

 Ehuddnant ; for, if the axes of those folds be prolonged southwards, 

 they pass well to the east of Craig-y-Ceffyl. 



The grit-group strikes steadily southwards from this locality for 

 about a mile, beyond which I have not followed it. When last 

 seen, it was striking along the steep slope of Llethr Brith and 

 Penlan-fawr towards Pentre Briwnant ; but, as has been men- 

 tioned already, it does not make its appearance along the road at 

 Cwm Ystwyth. A good deal of detailed work might be done on 

 that complicated mining district, which would take up more time 

 than I had at my disposal. 



