378 Reports <& Proceedings — Geological Society of London. 



lineage: (1) Kelestoma elongatum, Marsson, with an incrusting 

 asty ; (2) a new species, with a bilaminar, erect asty; (3) K. scalare, 

 Lang, with an erect, cylindrical asty. There is, in this series, 

 a slight catagenetic decrease in the number of costae, and the 

 avicularian aperture becomes somewhat more pointed. The genus 

 occurs in the Senonian, zone of Belemnitella mucronata, in the island 

 of Rugen. 



Morphasmopora, unlike Kelestoma, retains a small number of costas 

 and a short oecium ; but the thickness of the proximal apertural 

 spines, which are hai'dly recognizable as such, is enormously 

 increased; the thickness of the bifid apertural bar is also increased. 

 In Morphasmopora hrydonei, Lang, there are four circum-apeitural 

 avicularia; and the proximal apertural spines and the apertural bar, 

 though enormously developed, are not so large as in M. julces-lrownei 

 (Brydone). The latter species has fewer costse than the former, and 

 but one pair of circum-apertural avicularia. There are also differences 

 in the intercecial and interstitial secondary tissue of the two species. 

 3L brydonei occurs in the island of Eiigen and M. jukes-broumei at 

 Trimingham ; both from the Senonian, zone of Belemnitella mucronnta. 



(2) " The Greology and Genesis of the Trefriw Pvrites Deposit." 

 By Eobert Lionel Sherlock, D.Sc, A.R.C.Sc, F.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 graptolites 

 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 Caduant Shales of the Conway Mt. succession : tiiat 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 of 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, probably 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, 



