﻿Vol. 66.~] IN THE LEAD AND ZINC VEINS OF OEEAT BKITAIN. 303 



The silver- content of the galena (4 to 15 ozs. per ton) is much 

 higher than that of sedigenetic ores, and the restriction of the ore 

 in the veins to certain horizons is clearly due to the influence of 

 different strata on ore-deposition, and not to any greater abundance 

 of disseminated metals in these strata. The ore-deposits are totally 

 unrelated to drainage-lines, topographic slopes, synclinal basins, or 

 other features such as are often found to be determining factors in 

 the occurrence of the American deposits. 1 These features were 

 examined during the present investigation in North Derbyshire, 

 with the aid of the geological and topographical maps, but the 

 results were negative or inconclusive. On the other hand, both in 

 that district and in Flintshire, the deposits show a tendency to 

 concentrate beneath anticlinal arches, especially when these are 

 or have been covered by a ' blanket' of impervious shales. Further, 

 the localization of ores in certain tracts, which might suggest a 

 greater original abundance of disseminated metals in these tracts, 

 is not at all marked in the British limestone districts ; and, when 

 it does occur, it is seen to be dependent on the localization of 

 fissures or on the presence of strata favourable to ore-deposition. 



It is, therefore, concluded that the lead and zinc ores in the 

 limestone districts, like those in the older slates and granites, have 

 been brought into the fissures by solutions ascending from deep- 

 seated sources, at a period subsequent to the deposition of the rocks. 



It should here be noted that some limestone areas, particularly 

 North Derbyshire, are characterized by abundant small and scattered 

 deposits of ore occupying solution-cavities and enlarged joint- 

 planes ('flats' and 'pipes'), the galena having generally a very low- 

 silver-content. These deposits are comparable in all respects to the> 

 sedigenetic type, and are probably the result of later re-arrange- 

 ment and concentration by circulating waters — the ores found 

 in them being derived chiefly from the ores previously deposited in 

 the adjoining veins, and possibly to some extent from the traces 

 of disseminated metals in the surrounding rocks. 



Barium. — The most noteworthy feature in the occurrence of 

 barytes in these veins is its irregular distribution. Thus it is abun- 

 dant in the Leadhills, the Lake District, Shropshire, and North 

 Derbyshire, while it is rare in the Isle of Man, Flintshire, Cardigan- 

 shire, and Devon. The importance of barium as a minor constituent 

 in the rocks of the earth's crust is now widely recognized, and its 

 original source is generally considered to be in the felspars and micas 

 of the igneous rocks. 2 It is most probable, therefore, that the 

 barium minerals in the veins were concentrated in the first place 

 by magmatic processes, along with lead, zinc, and fluorine, and 

 that the barium was similarly brought to its present position by 

 solutions ascending from deep-seated igneous rocks. 



1 H. F. Bain, ' Zinc & Lead Deposits of the Upper Mississippi Valley ' Bull 

 U.S. Geol. Surv. No. 294 (1906) pp. 66-70. 



2 F. W. Clarke, ' The Data of Geochemistry ' Bull. U.S. Geol. Surv. No 330 

 (1908) p. 14. 



Q. J. G. S. No. 262. x 



