﻿Vol. 66.] IN THE LEAD AND ZINC VEINS OF GREAT BRITAIN. 321 



In general, then, the vertical succession of ores in the veins is in 

 agreement with their paragenetic relations determined above, 

 relations which are likewise supported by the relative solubilities 

 of the sulphides as determined by Prof. C. Doelter and U. A. Binder. 1 



VI. Influence of the Country-Kock on Ore-Deposition. 



The effect of different strata on the distribution of ore in the 

 veins is strikingly shown in many districts. Each area, however, 

 has its own features, determined by local circumstances, and what 

 may be the general rule in one district, sometimes fails in its 

 application to another district. It must be pointed out that 

 enrichments at vein-intersections, accompanied by vertical or 

 pitching shoots of ore, are frequent. These have not been specially 

 investigated in this work, as, owing to the inaccessibility of so 

 many of the old mines and workings, there are not sufficient 

 opportunities to make extended observations. The general facts 

 and principles governing the effect of the country-rock are here 

 summarized, followed by an account of some experiments dealing 

 with the chemical aspect of the problem. 



Veins in the older Palaeozoic slates and associated rocks show 

 similar features throughout. Where, as is generally the case, 

 strata of different character succeed one another vertically, the 

 relative abundance of ore is dependent on the character of the 

 fissure, and this, in turn, has been determined by the physical 

 properties of the different strata which it intersects. At the 

 Leadhills, the fissures are well defined and the ore good in the 

 grit-beds of the Grlenkiln-Hartfell Series, while impoverishment 

 occurs when the fissures pinch or scatter in the associated shale- 

 beds. The same relation holds in the Shelve district of Shropshire, 

 where, by the alternation of slates and shales, there have been 

 formed typical ore-horizons in the slates, separated by unproductive 

 stretches in the shales, 2 a condition analogous to that described by 

 Mr. C. W. Purington from San Juan (Colorado). : ' Similar features 

 occur in the Isle of Man 4 ; while in Cardiganshire, the softer 

 shale-beds, the ' metalliferous slates ' of Walter Keeping,' are more 

 favourable strata than the hard and resistant grits. In the Lake 

 district the fissures in the firm Borrowdale ash-beds, as at Green- 

 side and Coniston, are more regular, and the ore more evenly distri- 

 buted, than in the softer Skiddaw Slates, where, as at Thornthwaite 

 and Threlkeld, the veins are irregular and patchy. In all these 

 and similar cases the essential factor has been the nature of the 

 fissure or circulation-channel, which is best defined in those strata 

 that admit of the cleanest fissuring. The effect is a relative one : 



1 Tschermak's Min. Petrol. Mittb. n. s. vol. xi (1890) p. 319, & vol. xii (1891) 

 p. 332. 



2 D. C. Davies, 'Metalliferous Minerals & Mining' London, 1901, p. 222. 



3 ' Economic Geology ' vol. i (1906) p. 129. 



4 G. W. Laraplugh, ' Geology of the Isle of Man ' Mem. Geol. Surv. 1903, 

 p. 491. 



5 Quart, Jo urn. Geol. Soc. vol. xxxvii (1881) p. 141. 



u W. W. Smyth, Mem. Geol. Surv. vol. ii. pt. 2 (1848) p. 655. 



