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PROCEEDINGS OP MADISON MEETING. 



glaciation, the deposits being confined to the driftless area. While the glaciers swept 

 southward on either side, the lead region escaped and has since early geologic time 

 been subject to atmospheric influences. As a result, there is a great accumulation 

 of decomposed rock in place in an highly oxidized condition: That these products 

 of decomposition have been leached by surface waters there can be no question, 

 and we may regard a portion at least of the upper mineral deposits as derived from 

 a considerable thickness of formerly overlying rocks, now concentrated in the re- 

 siduum of their decay or redeposited in the crevices and cavities of lower strata. 

 The formerly overlying shales of the Hudson river or Cincinnati period of which 

 we now have remnants only in the mounds along the southern margin of the zinc 

 and lead region, may have been, in part at least, the original depository of diflused 

 sulphides of zinc and iron. Such sulphides by long exposure to oxidizing condi- 

 tions by the percolation of atmospheric water before and during the glacial period 

 would form soluble sulphates which by contact with the underlying limestones or 

 dolomites would be reprecipitated as carbonates, or, possibly, if brought in contact 

 with active deoxidizing conditions, as sulphides. 



Conditions similar to these hypothetical conditions are found at ancient Laurium, 

 where, according to M. Huet,* deposits of zinc ore penetrate limestone strata below 

 overlying schists. Interesting cross-sections of these deposits are given by Huet 

 and are cited by Daubree.f 



Professor A. H. Winchell has suggested J that the sulphide ores of the AVisconsin 

 lead region have been derived from formations formerly overlying the Trenton 

 even as high in the series as the Cretaceous, which he believes once covered the 

 lead region and have been swept away. He thinks that the ores were deposited 

 from the ocean in the Cretaceous age and found their way downward through the 

 strata to their present horizon. 



The existence of the ores in the driftless area and their absence in the same rocks 

 beyond is good evidence of their superficial rather than deep-seated origin ; that 

 they have accumulated from above downward rather than from below upward, and 

 that these ores were diffused in the mass of the preexisting rocks. If the ores 

 had been formed by the upward flow of solutions depositing their metallic sul- 

 phides along the walls of the fissures and crevices we should expect to find the 

 roots of the deposits, so to speak, in the glaciated strata. 



General Distribution of Zinc in Sea-water in Silurian Time. 



It does not appear necessary to invoke the aid of favoring currents of sea-water 

 speciall}'' charged with metallic solutions, or the derivation of such solutions from 

 remote and more favored shores, to enable us to accept the idea of the derivation 

 of the zinc and lead ores from the ocean. 



The fact that the older limestones and dolomites of both America and Europe, 

 and probabl}^, we may say, of the world, appear to be the special repositories of 

 zinc and lead is evidence of a world-wide and not local distribution of the salts of 

 zinc and of lead at that time. We find a series of remarkable workable deposits 

 of zinc ore in the Appalachians stretching at intervals from northern New Jersey 

 into Pennsylvania, Virginia and Tennessee. These deposits are either in the lime- 



*Huet: Mernoire de la Soeiete des ingenieurs civils, 1878-188C. 

 t Daubree : Les Eaux Souterraines, vol. iii, pp. 104-106, 

 I Iron Ores of Minnesota, p. 153. 



