﻿Yol. 66. ,] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 165 



have also obtained a single specimen of Calymene parvifrons. 

 These are the only definite Arenig fossils yet discovered in the 

 Ynyscynhaiarn district. 



(2) The Western Slate Rocks. 



The basal grit just considered is overlain in the Criccieth district 

 by a thick series of slates, which, although they occupy a great 

 area, offer little opportunity for the making out of their succession. 

 Near Portmadoc the possible slate-outcrop is deeply buried beneath 

 the alluvium of the Afon Glaslyn. 



Along the foot of the Tremadoc scars the slates are so mixed up 

 in the great thrust-faults which I shall describe later (p. 167) that 

 I am unable to disentangle their succession, and in the Criccieth 

 country (Hoel-bach and Braich-y-saint), where the outcrop is widest, 

 the only exposures are the low hummocks of rock which protrude 

 through the scanty soil of well- cultivated land. 



In this western district I have at three places measured an 

 apparent thickness of more than 1000 feet of rock ; but, knowing the 

 complexity of the strike-faulting in the grit-beds below, I suspect 

 isoclinal folding from the west, and am little disposed to attach 

 importance to the measurement. 



The lowest slates, which adjoin the grit, are generally dark, and 

 contain bands of coarse rock with kaolinized felspars (probably of 

 volcanic origin). From 50 to 200 yards west of the grit-outcrop, 

 the slates are very dark in colouring, but soft and earthy in texture. 

 These generally refuse to break along their bedding-planes, and 

 crush when struck with a hammer. In the Ceunant-du (a strange 

 glacial overflow-channel on the south-western slopes of Moel-bach) 

 the black rocks are seen to pass up into silky, blue-grey, spotted 

 slates, which are well exposed, and have yielded a few tuning- 

 fork graptolites that seem to belong to an early form of Didy- 

 mograptus murchisoni. 



At the foot of the Ceunant and along the hill-slopes to the old 

 Gloddfa slate-quarry, which is now used as a reservoir for the 

 Criccieth water-supply, the slates become paler and rather promi- 

 nently banded. Some of the bands there are ashy and almost flaggy 

 in texture, and, unlike the slates between them and the basal grit, 

 take on a bright rusty tone as they weather. 



The slates quarried were soft and almost too well cleaved ; they 

 are dark blue-grey in colour, showing rusty bands as they weather, 

 and break with a lustrous silky sheen. They are exceedingly like 

 the Llanvirn Slates of Llanfihangel-y-pennant and the northern 

 Snowdon slate-belt. 1 



The exposures of beds higher than these are all unsatisfactory, 

 but it would seem that there is a further series of dark-blue mud- 

 stones, and of very shivery blue shales, before we come to the sooty, 

 mica-spangled, black beds which underlie the felsites of Moel 

 Ednyfed and Ystum-cegid. I have been unable to find any fossils 

 in these higher slates. 



i Eep. Brit. Assoc. 1903 (Southport) p. 665. 



