164 



WALTEK KEEPING ON THE 



but they die out to the north and south of these localities. No 

 fossils have yet been detected in this series in Cardiganshire, except, 

 perhaps, in the boulder-block at Gogerddan, near Aberystwyth. 

 Stratigraphically they lie above the Metalliferous slates ; and these 

 latter are proved, in the Llanbr}'nmaer, Dovey, and other areas, to 

 be inferior to the Tarannon shales. Therefore, with reference to the 

 Noedd Grug section, they must either represent the uppermost part 

 of the Llandovery rocks or a special gritty development in the Taran- 

 non shales. I adopt the latter view for the following reasons : — 

 because of (1) their dissimilarity to the grits (Llandovery grits) 

 which underlie the Tarannon shales at Llanbrynmaer, Llan y 

 Mowddwy, and elsewhere ; (2) their association with pale slate rocks 

 similar to the Tarannon shales of the Corwen area ; and (3) the oc- 

 currence of such a group of rocks, the Gala grits*, in this position 

 in the south of Scotland. Moreover, in the Tarannon district the 

 first appearance of such a development of grits in the Tarannon 

 shale is already seen in the numerous thin grits, with contorted 

 structure, of Tarannon Hill, Llanbrynmaer. The Ehyader pale 

 slates thus appear also to belong to the true pale slate or Tarannon- 

 shale series. 



(B) The Metalliferous slates, which are enormously developed, 

 spreading over a very wide area of Mid "Wales. They maintain one 

 general lithological character of hard, pale, shaly slates, also containing 

 a zone of pale slates. The group is generally plumbiferous. 



(C) The Aberystwyth grits may be taken as an arenaceous de- 

 velopment of the Metalliferous slates, in its lower part. Like the 

 Plynlimmon grits they die out to the north and south, their southerly 

 attenuation being well exhibited around Llangrannog. They dip 

 persistently under the Metalliferous slates ; and the truth of this dip 

 is proved by the position of the contorted raised structures upon the 

 under surfaces of the grits f . 



In the Dovey valley the Aberystwyth Grits are represented by 

 gritty beds of a similar character ; but here they appear to belong 

 to a slightly different horizon, namely to the upper part of the 

 Metalliferous slates ; and the same is true of the grits exposed in 

 the deep Talerddig cutting, Llanbrynmaer. The grits and conglome- 

 rates of Talieris, west of Llandeilo, very probably belong to the 

 horizon of the Aberystwyth beds. In the Noedd-Grug section these 

 beds, together with the Metalliferous slate, are represented by the 

 Llandovery group of the Geological Survey, probably the Upper and 

 part of the Lower Llandovery ; but from the fossil evidence we can- 

 not recognize the lowest part of the Llandovery Group (Lower Eirk- 

 hill) in Central or West Cardiganshire. Part of the lower set of 

 slates beneath the grits, described in the Cardigan and Dovey areas, 



* Mr. Lapworth, F.G.S., writes me that " all the palreontological affinities 

 of the Gala beds are with the Tarannon shales, with which, and not with the 

 Upper Birkhill beds, they must eventually be connected." 



t I have confirmed this test by examining the beds of Lingula-flags, Trema- 

 doc and Bala rocks of South Wales, where these curious worm-like, fucoidal, 

 and irregular prominences are found uniformly upon the under surfaces of the 

 grits. 



