114 DR. K. L. SHERLOCK OX THE [vol. lxxiv, 



after the igneous mass was injected, and therefore be of Bala age 

 or very little later. This is of interest, because A. M. Finlayson L 

 expressed the opinion that the ore-deposits of this country 

 practically all originated at one of four periods: (1) Pre- Cambrian; 

 (2) Post- Silurian ; (3) Post- Carboniferous ; and (4) Post- Creta- 

 ceous and early Tertiary. Here, however, we have an ore-body 

 which was not formed at any of these periods. Finally, on the 

 above theory, the pisolitic iron-ore was already in existence before 

 the intrusive mass was introduced in late Bala times, a confirma- 

 tion of the sedimentary origin of the iron-ore, and opposed to 

 the theory that the pisolitic ores are really of the nature of fissure 

 phenomena. 3 



Discussion. 



Dr. A. H. Cox remarked on the interest of the paper from the 

 stratigraphical and the economic standpoint. With regard to 

 the suggestion that the pyrites originated from the alteration of 

 a pisolitic iron-ore, it was important to remember that beds with 

 Nemagraptus gracilis and Awplexograptus arctus, which in South 

 Wales comprised only a small thickness of strata, were represented 

 in North Wales by 2000 to 3000 feet of rock, owing to the in- 

 coming of a thick volcanic series. Beds below the volcanic series 

 might well belong to the same, or nearly the same, graptolite-zone, 

 according to the South Wales correlation, as beds above, yet the 

 difference in stratigraphical position was obvious. The Lower 

 Cadnant Shales of Conway, with which the graptolitic shales at 

 Trefriw were correlated by the Author, appeared to represent the 

 arctus beds of South Wales, that is beds just above the Mydrim 

 Limestone = zone of Nemagraptus gracilis, and they occurred 

 directly above the acidic — SnoAvdonian — volcanic series. On the 

 other hand, the pisolitic iron-ores of North Wales, while apparently 

 occurring in the zone of N. gracilis, were invariably found, so far 

 as he (the speaker) knew, below the acid volcanic rocks. He 

 knew of no occurrence of pisolitic rocks in the Cadnant Shales or 

 in other beds above the volcanic rocks, an,d he enquired whether 

 such pisolitic iron -ores had actually been discovered at Trefriw or 

 at the corresponding stratigraphical horizon elsewhere in North 

 Wales. 



Prof. C. G. Cullis congratulated the Author upon his interesting 

 communication. He also took this opportunity of offering con- 

 gratulations to the Geological Survey, as a whole, upon the 

 excellent work that they had done in economic geology, both 

 officially -and unofficially, since the beginning of the War. 



The origin of pyritic deposits was of much interest. Genetically 

 they were of two main types- — low-temperature deposits of sedi- 

 mentary origin, and high -temperature ones of igneous origin. The 



1 ' The Metallogeny of the British Isles ' Q. J. G. S. vol. lxvi (1910) pp. 281- 

 98 & pi. xxiii. 



2 W. G. Fearnsides, Rep. Brit. Assoc. (Leicester, 1907) 1908, p. 510; and 

 Geol. Mag. dec. 5, vol. iv (1907) p. 422. 



