﻿Vol. 66.] IN THE LEAD AND ZINC VEINS OF GREAT BRITAIN. 327 



and porosity, and secondly by its chemical composition : limestones 

 and dolomites being most favourable for the deposition of lead and 

 zinc ores. Subsequent denudation, to the extent of nearly a half of 

 the original height of the veins, has been accompanied by much 

 secondary alteration, the chief secondary processes being the 

 enrichment of silver and the depletion of blende in the upper 

 parts of the veins, and the formation of metasomatic deposits of 

 calamine. These secondary processes have been effective to depths 

 of over 600 feet. 



In connexion with these researches on British ore-deposits, I 

 wish to express my warmest thanks to Prof. R. A. S. Eedmayne, 

 Chief Inspector of Mines, to Dr. John Home, and to the managers 

 of the different mines visited, for opportunities of doing field- 

 work ; and to Prof. W. W. Watts for his advice and criticism 

 in the laboratory and in the preparation of the results. 



Discussion. 



Dr. D. Mawson remarked that the paper specially interested 

 him, as he had just completed a detailed memoir dealing with the 

 genesis of the Broken Hill Lode. The importance of this latter 

 was witnessed by the fact that within the last twenty-five years, 

 ore to the value of .£60,000,000 had been won from it. 



In the more important cases mentioned in the paper just read, 

 the speaker was glad to hear that the Author regarded the genesis 

 of the ore as arising from deep-seated sources. His (the speaker's) 

 experience was confirmatory of the fact that the principal sulphidic 

 lead-zinc ores of the world were primary depositions along lines of 

 weakness, and originated from ascending, usually thermal, mineral- 

 ized waters. The Broken Hill Lode was a case in point, in which 

 he regarded the ore as an ultimate separation-product from a 

 bathylitic igneous intrusion, and in this manner was of the nature 

 of a pegmatite. The genetic connexion between pegmatites and 

 certain ore-deposits had recently been firmly established, and to it 

 special prominence had been given by Mr. Spurr. 



Dr. Cttllis congratulated the Author, and was inclined to support 

 the contention that, while the minerals of those British lead-zinc 

 lodes which are enclosed in limestone might possibly have been 

 derived from the country rock, the contents of lodes in killas and 

 other sedimentary rocks were of a more deep-seated origin, and 

 probably originated from some underlying igneous source. The 

 rapid diminution and disappearance of the lode-minerals in the 

 surrounding country with departure from the lode, referred to by 

 the Author, seemed to point to this conclusion ; so also did the 

 nature of the veinstones characteristic of lead-zinc veins — calcite, 

 dolomite, quartz, fluorspar, and barytes. The first three of these 

 could hardly be claimed to favour one of the two suggested modes of 

 origin more than the other; but the last two, in view of the frequency 

 of fluorine in the emanations of deep-seated igneous masses, and of 

 barium in the lime-bearing felspars of igneous rocks, were strongly 

 suggestive of an igneous origin. In claiming a deep-seated origin 

 Q. J. G. S. No. 262. z 



