﻿Yol. 66.~] ORE-DEPOSITION IN LEAD AND ZINC VEINS. 307 



modification of Brogger's method, 1 illustrate the changes in com- 

 position of two of the types analysed. 



The limestones have been affected in a manner altogether different 

 from the silicate-rocks. The analyses (E x & E 2 ) show that the 

 wall-rock alteration has here been silicification and dolomitization. 

 Microscopic examination of the altered limestone reveals the 

 development of abundant quartz grains, either in aggregates or as 

 scattered crystals in the limestone, while small rhombohedra of 

 dolomite are sometimes distinguishable. 



The want of uniformity in the changes observed in the different 

 rock-types examined is essentially due to the different mineral and 

 chemical composition of the rocks themselves, which must have 

 greatly affected the results. Thus, a solution low in alkalies would 

 tend to extract potash or soda from a rock rich in these constituents, 

 while the same solution would have a reverse effect on limestone. 

 Such causes, bringing into operation the principle of relative mass 

 of interacting constituents, must make the conditions complex, and 

 prevent uniform changes in a wide district of varied country-rocks. 



As a general rule, it appears that in the silicate-rocks the charac- 

 teristic changes have been a concentration of potash, lime, and 

 carbon dioxide, and a leaching of silica, iron oxides, magnesia, and 

 soda, themineralogical effects being sericitizationand carbonatization. 

 In the carbonate-rocks silica has been generally introduced, and 

 some of the lime has been replaced by magnesia, the changes here 

 being, in other words, silicification and dolomitization. The results 

 are in accordance with the pioneer researches of Waldemar Lind- 

 gren. 2 The processes of alteration indicate that the vein-forming 

 solutions carried carbonates of the alkalies (chiefly potash) and of the 

 alkaline earths (chiefly lime). This conclusion is supported by the 

 great preponderance of calcite as a gangue-mineral in the veins. 



The chemical composition of the solutions is indicated in the 

 second place by the minerals which they carried. The lead and 

 zinc may have been transported, either as sulphides dissolved in 

 alkaline sulphides, or as oxidized salts (carbonates or sulphates) 

 dissolved in alkaline bicarbonates. The conclusions just reached 

 suggest the latter hypothesis ; but, on this view, it is very difficult to 

 account for the presence of the sulphides in the veins, since organic 

 matter is not generally a prominent constituent of the rocks, and 

 there are no other strong reducing agents present which could have 

 effected the wholesale reduction of oxidized salts of the metals to 

 the sulphides. The great quantity of sulphide-ore in the veins 

 seems to imply that the carriers of the metals must have been 

 alkaline sulphides, the efficiency of which as solvents of the metallic 

 sulphides has been shown by Prof. C. Dcelter 3 and others. 



1 W. H. Hobbs, ' Suggestions regarding Classification of the Igneous Rocks ' 

 Journ. Geol. Chicago, vol. viii (1900) p. 1. 



2 ' Metasomatic Processes in Fissure- Veins ' Trans. Amer. Inst. Min. Eng. 

 vol. xxx (1900-1901) p. 578. 



3 Tschermak's Min. Petrogr. Mittheil. n. s. vol. xi (1890) p. 319. 



