THEORIES OF ORE DISPOSITION JIISTORICALLY CONSIDERED. 329 



villo l>y Emmons, on Eurek.i by Curtis, and on tho copper-bearing 

 rocks of Lake Superior by Irving — monograi)liic studies which consti- 

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

 established ITnited States Geological Survey. It was the expectation 

 of (liose Avho planned this Avork that when all the important mining 

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

 snflici«Mit store of well-ascertained facts regarding ore deposits would 

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

 more firmly grounded on a basis of well-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 tliat. 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 Von 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 ultinuite source there is some 

 diil'erence of opinion, all are agreed that the concentrations which 

 produced the workable 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 j)ercolating waters had derived their metallic contei:wbs 

 from some of these eruptive rocks, which careful tests had shown to 

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

 derivation had an advantage over that of indefinite depth appealed to 

 Iw the ascensionist or hydrothermal school, inasmuch as it admitted 

 some sort of experimental proof, indirect though it Avas, and because 

 at the depth at which the rocks might be supposed to be essentially 

 richer in metals than those found at the surface, cracks sufficiently 

 open to admit a free floAV of thermal waters were considered impos- 

 sible under the conditions of pressure assumed to exist there. This 

 view was called a lateral-secretion theory, though it differed essen- 

 tially from that of Sandberger. in that the derivation of the vein 

 minerals was not restricted to the immediate wall rocks (Nebenges- 

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

 ized as another form of the ascension theory. The circulating waters 

 which had brought in the vein materials were assumed, though not 

 always explicitly, to be of meteoric origin — Avaters Avhich originally 

 descending from the stirface had become heated either in contact Avith 

 igenous rocks or by the internal heat of the earth, and gathering up 

 mineral matter in their journey had redeposited it Avhen conditions 

 favored preciDitation rather than solution. The natural channels 



