112 DB. E. L. SHERLOCK ON THE [vol. lixiv, 



genesis of pyritic deposits presents great difficult}". While for- 

 merly they were all classed together, investigation during the last 

 few years has shown that such simplicity does not exist, but that 

 such deposits may be of different origins. The authors conclude 

 that some are magmatic segregations, others are due to contact- 

 metamorphism, others are lodes, and, finally, some are of sedi- 

 mentary origin. 



In the case of Trefriw the theory of magmatic segregation at 

 once breaks down, for pyrites is considerably denser than the 

 diabase and ought, on that hypothesis, to be found below the 

 igneous rock, or at least in hollows in it, instead of forming a 

 sheet of nearly uniform thickness over the top. 



Ore-bodies formed by contact-metamorphism, 1 in a large majority 

 •of cases accompany acid intrusions, are generally in more or less 

 impure limestones, and are usually associated with many meta- 

 morphic minerals, such as garnet, pyroxene, etc. None of these 

 ►conditions apply at Trefriw. A more important objection to a 

 •contact origin is that ores so formed are found within the zone of 

 metamorphism : we have seen that at Trefriw the metamorphic 

 effect is of the slightest, and is confined to a band of shale not 

 more than a foot thick. 



There is no ' lode ' at Trefriw, in the sense in which the word is 

 used hy Berschlag, Vogt, & Krusch {op. cit. p. x), that is, as 

 ■an ore-deposit of tabular shape occurring along a fissure or other 

 line of disturbance. 



There remains the possibility of the pyrites being of sedimentary 

 •origin. At the horizon of the zone of Nemagraptus gracilis 

 occur most, if not all, of the pisolitic iron-ores of Anglesey, 

 Carnarvonshire, and Merioneth. 2 It is at least a remarkable 

 ■coincidence that the Trefriw pyrites also occurs at this horizon. 

 I think that the origin of the pyrites was as follows. The igneous 

 rock was intruded beneath a bed of pisolitic iron-ore, and the heated 

 waters from the intrusion, although unable to do more than 

 harden shale at the contact, were nevertheless able to transform 

 the iron-ore into pyrites. A certain amount of pyrites would 

 probably be present in the iron-ore, for pyrites is a common 

 constituent of pisolitic iron and not infrequently is abundant 

 enough to render the pisolites valueless for smelting purposes. 

 Moreover, in South Wales, shales of the same age are markedly 

 pyritous, although they^ contain no pisolitic iron-ore. 3 But the 

 bulk of the sulphur would be provided from the igneous mass in 

 the form of sulphuretted hydrogen. The pisolites, at the outcrop, 

 •contain more or less limonite with much silica, etc. ; but this is 



1 F. Berschlag, J. H. L. Vogt, & P. Krusch, op. cit. p. 349. 



2 T. C. Nicholas, ' The Geology of the St. Tudwal's Peninsula (Carnarvon- 

 shire) ' Q. J. G. S. vol. lxxi (1915) pp. 125-26. Also E. Greenly, in a memoir 

 on Anglesey, now in the press, and others. 



3 T. C. Cantrill & H. H. Thomas in ' Geology of the South Wales Coal- 

 field. Part X: The Country around Carmarthen' Mem. Geol. Surv. 1909, 

 p. 45. 



