105 DR. tt. I». SHERLOCK' COf THE [vol. lxxiv, 



4. The Geology and Genes is of the Tbefriw Ptjutes Deposit. 

 By Robert Lionel Sheklock, D.Sc, A.K.C.Sc, F.G.S. 



(Read June 5th. 1918.) 



The mass of iron pyrites worked at Cae Coch, Trefriw (Carnarvon- 

 shire) appears to be unique in some ways, so far as Great Britain 

 is concerned, consequently its geological characters are of some 

 interest. 



In August, 1917, Mr. T. C. Cantrill and myself visited Cae Coch, 

 and as a result of what we saw I spent seven days in the following- 

 month in mapping geologically the area around the mine. In this 

 work I received all possible assistance from Mr. E. J. Morris, 

 F.G.S. , the Manager of the mine, to whose energy the successful 

 development of the deposit is due. To him I wish to return my 

 thanks, and also to Dr. H. H. Thomas for petrological notes and to 

 Mr. John Pringle for naming the fossils. 



The Geological Survey Map (Old Series 78 S.E.) published in 

 1852, shows a fault, or thrust, at the position of the ore-body, 

 separating Bala Beds above from Contemporaneous Felspathic 

 Trap (engraved F) below. The presence of p} r rites is not recorded, 

 nor is it mentioned in the accompanying memoir, probably because 

 it is not seen at the outcrop owing to weathering. At the 4th 

 milestone from Llanrwst the map shows a boundary-line, appa- 

 rently a fault or thrust, bringing in the same ashes on the 

 north, against Bala Beds on the south. In the memoir by 

 A. C. Ramsay, 1 Ave read that north of the 4th milestone from 

 Llanrwst volcanic ashes form all the high ground for about 

 a mile in width, as far as Llanbedr. The map, however, shows 

 the southern part of the mass as contemporaneous felspathic trap, 

 as at the mine. 



The nearest area of similar rocks that has been mapped in 

 recent years is at Conway, at the mouth of the valley in which 

 Trefriw lies. Miss G. L. Elles 2 has published an account of 

 that region, and, so far as can be inferred from the small area 

 now described, the rocks at Cae Coch may be correlated with the 

 Lower Cadnant Shales and Conway Volcanic Series of the Conway 

 •district. 



The ore-body is a seam of iron pyrites situated in the steep 

 western slope of the Conway Valley, about a mile north of Trefriw 

 village, where the hill-top rises precipitously to about 860 feet 

 above the river-flat. The ore runs obliquely up the hillside, with 

 an apparent dip of about 27° on the average, rising northwards. 

 The outcrop, however, is not quite straight, owing to some small 

 but sharp folds. Below the large old workings, now called Adit 



1 ' The Geology of North Wales ' Mem. Geol. Surv. 2nd ed. (1881) p. 135. 



2 ' The Relation of the Ordovician & Silurian Rocks of Conway (North 

 Wales) ' Q. J. G. S. vol. lxv (1909) pp. 169- 94 & pi. viii. 



