GEOLOGY OF CENTRAL WALES. 



161 



greatly swelling out the surface-area of the Metalliferous-slate 

 series. 



Minor foldings, also with north-and-south axes are infinitely 

 abundant over the whole district, with frequent inversions which 

 obscure the original order of the rock-beds. 



Included in these foldings are four principal crescent zones of 

 grit, greywacke, and conglomerate, namely : — 1st, the western or 

 Aberystwyth grits; 2nd, the eastern grits — an imperfect line of 

 grits seen at Talieris, west of Llandeilo, and perhaps continued in 

 other grits East of Gwastaden. These I regard as the diminutive 

 representatives of the Aberystwyth grits. 3rd. The Plynlimmon 

 grits and conglomerates, with much associated pale slate ; and 

 4th, the grits and conglomerates of Cwm Elan and Gwastaden, which 

 are also associated with, and, indeed, included in, a great pale-slate 

 group, the Ehyader pale slates. The last two are upon the same 

 stratigraphical parallel, occupying synclinals. These grits I do not 

 regard as necessarily holding exactly the same horizon over wide 

 areas, they having been probably shingle banks over the old 

 sea-bottom. The pale slates seen in so many sections appear 

 to belong to two principal horizons — (a) the lower or Strata Elorida 

 pale slates in the lower part of the Metalliferous-slate group, and 

 (b) the Ehyader pale slates, associated with the Cwm Elan, Gwastaden, 

 and Plynlimmon grits. 



More distant Sections (see Table of Vertical Sections, p. 16-1). 



In order to work out the relations of these rocks to the other 

 lower palaeozoic groups, I have visited various districts where the 

 Cardiganshire series might be studied in association with rocks of 

 some definite and decided horizon — namely, amongst other places, 

 Llanbrynmaer, Dinas Mowddwy, Cardigan, X. Wales, &c. 



In the Llanbrynmaer area we find (1) an upper series of grey 

 grit and greywacke, often very felspathic and micaceous, cleaved, 

 and with fragments of fossils (the Denbighshire grits). These pass 

 down by gradual transition into (2) a series of pale blue and green, 

 grey and purple shales, with some darker shale, rab, and green grits, 

 which make up the true Tarannon shales ; some obscure fragments 

 of Graptolites have been found here. Again, there is no break, but 

 a simple passage between this series and (3) the lower series of 

 grits, greywacke, and dark shales which belong to the Cardiganshire 

 type, and contain the fossils of our Metalliferous group. To the 

 west of the Tarannon plateau we find the Metalliferous slates fully 

 developed in the Pennant valley, with the characteristic fossil species, 

 and the usual associated lead-mines of that group. 



The whole character of this Metalliferous slate is different from 

 that of the Tarannon shale ; and our evidence points uniformly and 

 decisively to the Metalliferous slate being a more ancient series, the 

 two groups being separated in this area by a zone of grits. The 

 Plynlimmon grits and associated pale slates, as seen in. Plynlimmon 

 and around Ehyader, appear to be wanting in this area, there being- 

 no well-developed grit series between the Llandovery group and the 



