part 1] IGNEOUS AND ASSOCIATED EOCKS OF LLANWETYD. 31 



Pembrokeshire. 1 The latter, so far as known at present, appear 

 more restricted in their distribution, and are "best known in the 

 Cader-Idris and Snowdon districts. 



It was natural to suppose that the Llanwrtyd rocks were of the 

 same age as those at Builth and belonging to the lower series. 

 We have shown, however, that the Llanwrtyd rocks are of Upper 

 Glenkiln (Lower Bala) age : that is, contemporaneous Avith the 

 Mydrim Limestone of South Wales. Now, the Snowdonian 

 Volcanic Series rests conformably on beds yielding an Upper 

 Glenkiln fauna, 3 and is overlain by beds containing large Diplo- 

 grapti which indicate a very low horizon in the Hartfell (Bala). 3 

 The volcanic rocks in the Bala country occup} r a similar position. 

 Miss Gr. L. Elles informs me that they are immediately overlain 

 by shales which yield exactly the same fauna as the beds 

 overlying the igneous series of Llanwrtyd. In the Bala country, 

 as at Llanwrtyd, the fossiliferous horizon is succeeded by a great 

 thickness of unfossiliferous black slates and shales. The evidence 

 is, therefore, conclusive, that the Llanwrtyd igneous rocks 

 are of the same age as the Upper Series of Snowdonia, 

 Cader Idris, and the Bala country. 



The presence of the remarkable ' spilite-breccia,' as well as the 

 rhyolitic flow-breccia, at Llanwrtyd seems to indicate that the 

 centre of effusion of the rocks was not far distant. It may have 

 formed an isolated centre of eruption on the sea-floor in Upper 

 Llandeilo to Lower Bala times. 



I am permitted, by the kindness of Prof. A. H. Cox, to give 

 some further details of correlation with the series in the Llan- 

 drindod-Builth area. The southern part of this area was investi- 

 gated by Mr. H. Woods in 1894, and he came to the conclusion 

 that the igneous rocks were, for the greater part, pre-Lower 

 Llandeilo in age. Dr. H. Hicks, in the course of the discussion on 

 the paper, stated that he had concluded that the contemporaneous 

 volcanic rocks were quite at the base of the Llandeilo Series and 

 mainly associated with the Llanvirn Beds — as in Pembrokeshire and 

 North Wales. The latter opinion has been confirmed by Prof. Cox, 

 who informs me that the volcanic series includes typical spilites, 

 that it rests upon beds yielding Didymograptus bijidus, and is 

 overlain by deposits of Llandeilo age A/ielding Ogygia buchi. 



The existence of two volcanic cycles in the Ordovician of Wales 

 is thus confirmed by the presence of representatives of the two 

 series in the Builth and Llanwrtyd districts respectively. The 

 suggested correlation of the Llanwrtyd rocks is shown in the 

 appended table (p. 32). 



1 A. H. Cox, ' The Geology of the District between Abereiddy and Aber- 

 castle (Pembrokeshire) ' Q. J. G. S. vol. lxxi (1915-16) p. 273. 



2 W. G. Fearnsides, ' Geology in the Field' Jubilee Vol. Geol. Assoc. (1910) 

 p. 798. 



:i G. L. Elles, Q. J. G.S. vol. lxv (1909) p. 193. 



