THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 329 



ville by Einmons, on Eureka by Curtis, and on the copper-bearing 

 rocks of Lake Superior by Irving — monographic studies which consti- 

 tuted an important feature in the phm of work hiid out for the newly 

 eistablished United States Geological Survey. It was the expectation 

 of those who planned this work that wdien all the important mining 

 districts of the United States had been thus exhaustively studied, a 

 sufficient store of well-ascertained facts regarding ore deposits would 

 have been accumulated to admit of the fornndation of a new theory 

 more firmly grounded on a l)asis of w^ell-established fact than any that 

 had yet been presented. 



It may be said of the deposits studied in the first decade of the Sur- 

 vey work that, in the form in which they were found, they were all 

 determined to have been deposited from aqueous solutions and to be 

 of later origin than the inclosing rocks. The lead and zinc ores of 

 the Mississippi Valley might have been included in Yon Groddeck's 

 contemporaneous class if, as assumed by Whitney and Chamberlin, 

 these metals had been deposited with the limestones at the time of 

 their formation; but while, as to this ultimate source there is some 

 dirt'erence of o])ini()n, all are agreed that the concentrations wdiich 

 produced the Avorkable ore bodies were of later date; hence it seems 

 more logical to consider them of later formation than the inclosing 

 rocks. 



In the case of the other deposits studied, which were found to occur 

 either in or in the innnediate vicinity of eruptive rocks, it was as- 

 sumed that the percolating waters had derived their metallic contei^ts 

 IVom some of these erui)tive rocks, which cai-eful tests had shown to 

 contain small amounts of the vaiMous materials of the deposits. This 

 derivation had an ad\antage over that of indefinite de])th appealed to 

 by the ascensionist or hydrothernial scliool, inasmuch as it admitted 

 some sort of experimental proof, indirect thougli it was, and because 

 at the depth at which the rocks might be su])posed to be essentially 

 richei- in metals than those found at the surface, cracks sufficiently 

 open to admit a free flow of thermal w^aters were considered impos- 

 sible under the conditions of ])ressure assumed to exist there. This 

 view was called a lateral-secretion theory, though it ditt'ered essen- 

 tiallv from that of Sandbergei-, in that the derivation of the vein 

 minerals was not restricted to the innnediate wall i-ocks (Nebenges- 

 teine) of the deposits. IndecHl, in a later discussion it was character- 

 ized as another form of the ascension theory. The cii-culaling waters 

 which had broiiglil in the \-ein matei-ials were assumed, though not 

 always explicitly, to be of meteoric origin — waters which oi-iginally 

 descending from the surface had become heated cither in contact with 

 igenous rocks or by the intei-nal heat of the earth, and gathering up 

 mineral matter in their journey had redeposited it when conditions 

 favored preciintation rather than solution. The natural channels 



