THE VERIFICATION PERIOD 23 



siderably modified by the important r6le that the process of metasomatic 

 replacement or substitution has been shown to have played in the forma- 

 tion of ore bodies. The idea of replacement had been suggested in the 

 conversion theories of the early speculators and more distinctly ex- 

 pressed by Charpentier. By the geologists of the second period it was 

 comparatively neglected, though in a few cases admitted as a subordinate 

 factor, especially in the formation of deposits in limestone. Even in 

 Posepny's frequently quoted studies of the lead and zinc deposits of 

 Raibl in Carinthia (1873), he admits this mode of formation only for the 

 oxidized ores, considering the sulphide ores to have been deposited in 

 open cavities. 



In America, Pumpelly first applied this process to thecopper deposits 

 of the Lake Superior region (1871), of which he says : " In at least very 

 many instances, if not in all, the deposition of the copper is the result 

 of a process of displacement of pre-existing minerals." 



Leadville and Eureka were the first large mining districts in which it 

 was proved that extensive ore deposits were entirely formed by meta- 

 somatic replacement of the enclosing rock, which in these cases was 

 limestone. Later observations showed that this form of deposit was not 

 confined to limestones, and that in fissure vein deposits, even in acid 

 rocks, metasomatic processes had often played an important part in 

 replacing by ore portions of the country rock which, under the old views, 

 might have been regarded as vein filling. The interest and importance 

 of this view were speedily recognized, especially by American geologists 

 and mining engineers, and while still novel, it was doubtless sometimes 

 applied without sufficient proof as an explanation of the formation of 

 certain vein deposits to the exclusion of that of the filling of cavities or 

 interstitial spaces. With the general introduction of the microscope 

 into the study of vein materials, however, a comparatively sure method 

 was provided of distinguishing the results of the two processes. The 

 process of verification has in this case resulted in the establishment of 

 the importance and increasingly wide applicability of the metasomatic 

 theory to the formation of ore deposits of all types. 



In the latter part of the decade Irving and Van Hise's studies of the 

 iron deposits of the Lake Superior region had demonstrated that they 

 had been deposited from solution in descending or meteoric waters, 

 whose downward course had been arrested by some impervious base- 

 ment — sometimes a dike, sometimes a bed in a synclinal basin — and 

 that during this time of stagnation their load of iron oxide had been laid 

 down as a metasomatic replacement of the enclosing rock, a descen- 

 sionist theory but of essentially modern type. 



In 1893 appeared the well known paper, " The Genesis of Ore De- 

 posits," by Posepny, for ten years professor of this branch of the science 



