﻿Vol. 66. ~] IN THE LEAD AND ZINC VEINS OF ORE AT BRITAIN. 325 



alteration and enrichment of the upper parts of the veins, expressed 

 in the formation of a superficial zone of oxidization, succeeded by a 

 zone of cementation, which finally passes down into the zone of 

 unaltered ores. The secondary changes which have taken place 

 indicate that the chemical processes of solution and cementation 

 have proceeded more rapidly than the physical processes of 

 denudation. 



In the zone of oxidation, above the permanent ground water- 

 level, secondary oxidized compounds are abundant, and have 

 received considerable notice, especially at the Leadhills and at 

 Caldbeck Fells. In the cementation-zone beneath, Prof. C. P. Tan 

 Hise's conception of a second concentration by descending waters, 

 as distinguished from the first concentration by ascending waters, 1 

 is well illustrated in the limestone districts. The characteristic 

 solution-cavities, that is, ' pipes ' and ' flats,' in limestone are clearly 

 the work of descending waters, and they have been more or less 

 filled with ores and gangue-minerals by the same agency. The 

 flats in the Flintshire area frequently contain masses of galena 

 embedded in loose sand derived from the disintegration of cal- 

 careous sandstone, while pipes are often similarly filled with more 

 or less detrital matter. In the Halkyn vein, at a depth of 

 600 feet, a large cavity over 20 feet in width and 60 feet high has 

 been dissolved out along the vein-fissure by underground waters 

 at a point of vein-intersection, with complete removal of both 

 ores and gangue. The depth of effect of the waters is similarly 

 indicated by the remarkable occurrence, in some Flintshire veins, 

 of tree-trunks to a depth of 600 feet. 2 Sulphides have been 

 abundantly redeposited below the ground water-level, and the many 

 crystallizations of galena and blende in vughs and cavities are 

 largely of secondary origin; while fluorspar has probably also been 

 re-arranged at the same time. The depth to which secondary 

 processes have extended varies between 600 and 1000 feet, 

 according to the permeability and extent of Assuring of the rocks. 

 That much of this work was accomplished before the Glacial Period 

 is evident from the occurrence, in some districts, of pre-Glacial 

 clays containing galena and ceru^site, which fill hollows and pipes 

 in the rock. The ' gravel-ore ' of Talargoch and other parts of 

 Flintshire, 3 containing waterworn fragments of galena and earthy 

 cerussite in a matrix of pre-Glacial clay and sand, is a further 

 remnant of the earlier denudation processes. 



One of the most important effects of descending waters has been 

 the enrichment of galena by silver in the upper horizons. The 

 appearance of native silver in the more argentiferous galena, 

 already described, is the result of secondary concentration. At 

 Hilderston, in Linlithgowshire, niccolite in the veins was argenti- 

 ferous to a depth of 70 feet, below which it quickly became 



1 C. R Van Hise, 'A Treatise on Metamorphism ' Monogr. xlvii, U.S. Geol. 

 Surv. 1904, pp. 1144-58. 



2 A. Strahan, 'Geology of Flint, Mold, & Ruthin' Mem. Geol. Surv. 1890 

 pp. 178, 190. 



; A. Strahan, 'Geology of Rhyl. Abergele, & Colwyn ' Mem. Geol. Surv 

 1885, p. 47. 



