Origin of the Iowa Lead and Zinc Deposits. — Leonard. 289 
other forms gave rise to gases which were doubtless very ef- 
fective in precipitating the metallic sulphides, which were thus 
deposited along with the sediments. 
Localization of the deposits. One fact in connection with 
these deposits is difflcult of explanation on the theory of oce- 
anic deposition, namely, the localization of the ore bodies. 
These bodies are confined to certain districts, outside of which 
the ground is nearly or quite barren, although as far as can 
be seen the conditions are quite as favorable. For example, 
the Dubuque mines are confined to an area of some twenty 
square miles, while to the north and south the Galena lime- 
stone carries no ore though every whei'e cut by numerous crev- 
ices favorable for its reception. The same is true of the many 
mining districts of Wisconsin and Missouri. The mines are 
noticeably grouped about certain centers while the surround- 
ing country is unproductive. The natural supposition would 
be that the minerals were everywhere equally disseminated 
through the rock and that they have been leached out and 
deposited in the lissures only in certain favorable localities. 
But this is hardly probable in view of the fact that the con- 
ditions are apparently just as favorable for the formation of 
ore bodies in the barren districts as in the productive ones. 
How then can we account for the localization of the depos- 
its on the generally accepted theory that the lead and zinc 
were contained in the oceanic waters and were distributed 
through the rocks at the time of their formation? 
Professor J. D. Whitney* over thirty years ago published 
an elaborate report on the lead region and discussed at con- 
siderable length the origin of the deposits. His views differ 
quite radicall}^ from those of Messrs. Owen and Percival, who 
had maintained that the metals were derived from great depths. 
Professor Whitney was the first to advance the theory that 
the metallic salts were held in solution in the waters of the 
ancient sea and were thrown down by organic matter or by the 
sulphuretted hydrogen arising from its decay. But no satis- 
factory explanation was given for the localization of the lead 
and zinc. 
Professor T. V. Chamberlinf attributed the original concen- 
*Geol. of Wisconsin. 1862. ~~~~ 
tGeol. of Wisconsin, Survey of 1873-1879, vol. rv, p. .529,1882. 
