THE VERIFICATION PERIOD 21 



Stelzner had called metasomatic deposits, said they could not be re- 

 garded as separate deposits, because they are only incidental phenom- 

 ena of the filling of cavities. 



As a means of obtaining a clear view of the whole field, Von Grod- 

 deck divided known deposits into types (54 in number), characterized 

 in the main by their varying mineralogical and lithological associations. 

 Of these, sixteen belong to his first subdivision, five to the second, 

 twenty-six to the third, and seven in part to the third and fourth, a 

 classification which he admitted must be considered but tentative, owing 

 to defects in existing knowledge which could be remedied only when 

 all mines could be studied on a monographic or exhaustive system. 



In America, though apparently unknown to Von Groddeck, such 

 monographic studies had already been made — that of the Comstock 

 lode by King (Fortieth parallel reports, 1870), of the Lake Superior 

 copper deposits by Pumpelly (Michigan Geological Survey, 1873), and 

 that of the lead deposits of the Mississippi valley by Chamberlin (Wis- 

 consin Geological Survey, 1873-1879). These were followed in the early 

 eighties by reports on the Comstock lode by Becker, on Leadville by 

 Emmons, on Eureka by Curtis, and on the copper-bearing rocks of lake 

 Superior by Irving, monographic studies which constituted an impor- 

 tant feature in the plan of work laid out for the newly established 

 United States Geological Survey. It was the expectation of those who 

 planned this work that when 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 accumu- 

 lated 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 Survey 

 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 enclosing 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 depos- 

 ited with the limestones at the time of their formation ; but, while as to 

 this ultimate source there is some difference 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 enclosing rocks. 



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

 either in or in the immediate vicinity of eruptive rocks, it was assumed 

 that the percolating waters had derived their metallic contents 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 



