CONCLUSIONS AS TO LEAD AND ZINC DEPOSITS. 31 



stones of the Trenton period or in close association with them. To the zinc and 

 lead regions of Wisconsin and Missouri we may add those of Arkansas and of 

 New Mexico, these last, at least, being in Silurian limestones. 



The zinc ores of Tennessee are found for the most part in the lower beds of 

 Trenton limestone or just below them in the magnesian limestones. The zinc ores 

 of Claiborne, Tennessee, occur at intervals for 12 or 15 miles along a great anticlinal 

 fold of the limestones.* The zinc deposits of Bald hill, on Powell river, Tennessee, 

 are in a dark-colored fetid limestonef of Lower Silurian or upper Cambrian age. 



In Europe, according to Burat, the ores of zinc used at the celebrated zinc smelt- 

 ing works of Belgium are obtained chiefly from dolomitic Silurian limestones, but 

 Daubree in his resume of the discussion of the action of mineralized waters in the 

 formation of masses of zinc ore indicates a much more general vertical range, for he 

 says, in substance, notwithstanding local differences the deposits of calamine pre- 

 sent striking analogies altogether independent of the age of the beds in which they 

 are spread, whether Cretaceous, as in Spain ; Triassic in Upper Silesia, the Duchy 

 of Baden and central France ; Carboniferous in Belgium and England ; Devonian 

 in some parts of Belgium, Prussia and Westphalia, or in Silurian, as in Sardinia. 



In the western portion of the United States some of the more remarkable large 

 bodies of zinc and lead ores, generally argentiferous, are either in or are closely 

 associated with the older limestones. This, it is thought, is good evidence in 

 favor of the contemporaneous origin of the sulphides of the metals with the mass 

 of the rocks in which we now find them, and consequently that they, like the 

 sediments, were derived from sea-water or were at least distributed by sea-water, 

 the chemical conditions then favoring the i^recipitation more than before or since. 



Conclusions. 



The chief points to which I desire to direct especial attention by this paper are : 



1. That faults and dislocations exist in the Wisconsin lead and zinc region, and 

 that these faults have a direct, though obscure, relation to the localization of the 

 mineral deposits, as claimed by Percival. 



2. That although it is not probable that the faulting planes gave vent to mineral 

 solutions from below, they probably permitted the outflow of fresh water or of 

 gases which acted upon the sea-water as precipitants of the metals and also as 

 destroyers of the animal and vegetable life in their vicinity, by the decomposition 

 of which organisms the accumulation of metallic sulphides in the rocks was pro- 

 moted and to some extent localized. 



3. That the significance of the thin oil-bearing shales at the top of the compact 

 limestones of the Trenton period and at the base of the Galena dolomites has not 

 been sufiiciently recognized. These shales are rich in petroleum, and give evidence 

 of sudden formation and of the attendant destruction of organic life. This "oil- 

 rock " is at the base of most of the zinc deposits, and appears to have acted both 

 as a retentive substratum, or floor of deposition, of blende and as a source of deoxi- 

 dizing and of sulphurizing gases which have determined the reprecipitation of the 

 zinc from sulphate solutions derived from the oxidation of the blende deposits 

 above the water-level. 



4. That the arrangement of the crevices indicates a shattering of the strata. 



*Safford : Greological Reconnoissance of Tennessee, 1855, p. 74. 



t Mining Magazine and Journal of Geology, second series, vol. i, p. 420. 



