L 488 ] 



LVI. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 428.] 



June 5th, 1918.— Mr. G. W. Lamplugh, E.R.S., 

 President, in the Chair. 



r pHE following communications were read : — 



1. ' The KelestorninaB, a Sub-Eainily of Cretaceous Cribri- 

 morph Polyzoa.' By William Dickson Lang, M.A., E.G.S. 



2. ' The Geology and Genesis of the Trefriw Pyrites Deposit.' 

 By Robert Lionel Sherlock, D.Sc, A.R.C.Sc, E.G.S. 



This pyrites deposit is worked at Cae Coch Mine, on the western 

 side of the Conway Valley (North Wales), about 1 mile north of 

 Trefriw. 



A band of pyrites, about 6 feet thick, and of considerable purity, 

 rests on the inclined top of a thick mass of diabase which is shown 

 to be intruded into the Bala shales that cover the ore-body. 

 The shales immediately above the pyrites are shown by the grapto- 

 lites contained to belong to the zone of Nemagraptus gracilis, and 

 are the equivalents of the Mydrim Limestone of South Wales and 

 of part of the Lower Cadnant Shales of the Conway Mt. succession : 

 that is, they are near the base of the Bala Series according to the 

 Geological Survey classification (Carmarthen Memoir, 1909). 

 Northwards the intrusive is bounded by an overthrust mass ol 

 volcanic ash, which itself is cut off by an east-and-west fault 

 against rhyolite, well seen in a roadside quarry and in the crags of 

 Clogwyn Mawr. Intrusions of dolerite of much later age, pro- 

 bably late Devonian, or Carboniferous, are found in the rhyolite, 

 and form the plateau above the mine, passing over shales, diabase, 

 ash, and rhyolite in turn. 



Pyrites deposits are classified by Beyschlag, Yogt, and Krusch 

 in four groups : — (1) Magmatic segregations, (2) formed by 

 contact-metamorphism, (3) lodes, (4) of sedimentary origin. 

 None of these modes of origin, however, will account for the 

 Trefriw pyrites. The conclusion arrived at is that the diabase was 

 intruded below a bed of pisolitic iron-ore. Hot water containing 

 sulphuretted hydrogen given off from the intrusion, combined 

 readily with the pisolites, which were in the form either of oxide 

 or of silicate of iron, and formed pyrites. The graptolitic horizon 

 at which the pisolitic ore occurs usually contains some pyrites, and 

 this would be added to that derived from the above reaction. The 

 pyrites was not formed by ordinary contact-metamorphism ; because 

 the intrusion is seen, at places where the pyrites* is absent, to exert 

 only a slight hardening effect on the shale. In North Wales 

 pisolitic iron -ore is known to occur in several places at the horizon 

 of Nemagraptus gracilis. From the mode of origin assigned above 

 to the pyrites it follows that the mineral is of Bala age, since it was 

 formed before the intrusion, itself of Bala age, had cooled. The 

 pisolitic ironstone must have been in existence in Bala times, and this 

 supports the idea that the ironstone is a bedded contemporaneous 

 deposit. 



