﻿Vol. 66.~\ IN THE LEAD AND ZINC VEINS OF GREAT BRITAIN. 309 



British limestone districts, a fact which is difficult of explanation 

 on the assumption of aqueous emanations direct from magmas. 



(2) The ore frequently fails at comparatively shallow depths. 

 The vertical range of the ores is, indeed, comparable to the depth 

 of the present underground circulation. 



(3) The silver-content of the galena in limestone-districts is 

 frequently much lower than in areas of the older rocks. This, 

 taken in conjunction with the known fact that the silver-value of 

 galena is reduced by prolonged solvent action of water or dilute 

 solutions, indicates that the ores have undergone much more trans- 

 port in these districts than in the slate and granite areas. 



(4) Extensive circulation is further put in evidence by the 

 structure of the deposits. The pipes, flats, and enlarged vein- 

 fissures which are typical of all the limestone-districts are clearly 

 the result of circulation by meteoric waters, and the rich deposits 

 of ore found in them have probably been formed by the same agency, 

 since, in many cases, the deposition of ore has been intimately 

 related to the formation of the cavities. Circulation furnishes the 

 key to ore-deposition in the limestone districts, and it seems 

 improbable that this circulation was effected so uniformly over 

 large areas by juvenile springs. 



(5) It is most probable that the main epoch of deposition of all 

 the lead and zinc ores of Britain was Permo-Triassic, a period 

 when the rocks were elevated and fissured, the climate was dry, 

 and erosion was rapid. These conditions were eminently favourable 

 to a deep circulation of atmospheric waters. 



(6) The importance of the underground circulation is indicated 

 by the occurrence of underground waters at the present day. The 

 springs in North Derbyshire are a good example : the Buxton 

 spring-water has a temperature of 82° F., indicating an undoubtedly 

 deep circulation. This water carries traces of lead and fluorine. 1 

 Further evidence comes from Flintshire, where the Halkyn drain- 

 age-adit, unwatering the mines on Halkyn Mountain to a depth of 

 250 feet below the summit of the mountain, delivers a million 

 gallons of water per day, and St. Winifred's Well, at Holywell, is 

 estimated to yield 4400 gallons of water per minute. 2 A sample 

 of water taken from the Halkyn drainage-adit was analysed for 

 lead in the course of this work, and found to carry from - 01 to 

 0'03 grains of lead oxide per gallon. The temperature of the 

 water was 52° F., and the solid contents (chiefly carbonates and 

 chlorides) amounted to 30 grains per gallon. 



(7) Lastly, there is the well-known fact that the toadstones of 

 North Derbyshire and the Whin Sill of Northumberland and 

 adjacent counties generally limit the downward extent of the 

 productive parts of veins which cut these rocks. There is in this 

 a further suggestion that the ores were to a large extent deposited 

 by underground waters circulating only above these impervious 

 layers. 



1 ' Geology of North Derbyshire ' Mem. Geol. Surv. 2nd ed. (1887) p. 115. 



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

 p. 222. 



