﻿328 OEE-DEPOSITION IN LEAD AND ZINC VEINS. [May I9IO. 



for the contents of these lodes, the Author would encounter certain 

 difficulties, such as the fact that lead-zinc lodes are so very much 

 more common in sedimentary than igneous rocks, and the further 

 fact, that the lode- walls in such cases practically never show signs 

 of any characteristic alteration. 



Prof. W. W. Watts congratulated the Author on the outcome of 

 a large amount of work in the field and in the laboratory. 80 far 

 as his own experience went, he had been unable to establish the 

 presence of barium in felspars in an area containing many veins of 

 barytes. 



The President (Prof. W. J. Sollas) expressed his satisfaction at 

 the advocacy of what seemed to him the common-seuse view of 

 lode-deposits. He did not think that the theory of lateral secretion 

 had ever taken root in British geology, and the observations of the 

 Author (so far as they went) were opposed to it. The order of 

 deposition of minerals was a difficult subject, and afforded room 

 for much subjective determination. In the Carboniferous Lime- 

 stone of the North of England it had been supposed that some lodes 

 were filled in by descending solutions. This was not impossible, if 

 lakes supplied by hot springs had extended over the limestone. 

 Barytes was associated with galena in lodes traversing killas, 

 and the probability was that both barium and lead had been 

 supplied by the same magma. 



The Author thanked the speakers, and in reply said that 

 the lead and zinc ores of the Mississippi and Missouri districts 

 were admitted by many workers to have been concentrated from a 

 disseminated condition : it seemed advisable, therefore, to examine 

 carefully any limestone district characterized by lead ores with a 

 low silver-content. In regard, to the order of deposition of the 

 sulphides, his experience had always been that the chalcopyrite 

 occurred in well-formed crystals enclosed in and wrapped round by 

 galena or blende. This was the case in ores from Conway, Coniston, 

 and Wicklow, where the ' bluestone ' found with the pyritic ores 

 enclosed occasional crystals of chalcopyrite. Ores from other parts 

 of the world showed the same relations, including galena- and 

 blende-bearing specimens from the pyritic masses of Southern 

 8pain. He had not in this paper considered the possible application 

 of the hypothesis of deposition of the ores jjer descensum. 



He agreed with Dr. Cullis that fluorine indicated a deep-seated 

 origin, at least in the districts studied ; and with another 

 speaker that it was frequently difficult to apply the results of 

 artificial laboratory experiments in explanation of the natural 

 phenomena. 



He was much interested in Dr. Mawson's observations on Broken 

 Hill. He had lately examined some Broken Hill ores by metallo- 

 graphic methods, which indicated that the garnet and rhodonite 

 had been more or less granulated and re-cemented by later galena 

 and blende. This agreed with the observations of Prof. E. Beck : 

 and the deposition of the sulphides at Broken Hill appeared to be 

 the last of the complex series of disturbances which had affected 

 that area. 



