THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 331 



metasomatic ivplacement of the inclosing rock, whicli in these cases 

 was limestone. Later obsecyations showed that this form of deposit 

 was not confined to limestones, and that in fissnre vein deposits, even 

 in acid rocks, metasomatic processes had often played an important 

 part in replacing- b}^ ore portions of the country rock which, under the 

 (.Id 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 

 W'as doubtless sometimes applied without sufficient proof as an expla- 

 nation of the formation of certain vein deposits to the exclusion of that 

 of the tilling of cavities or intei'stitial spaces. \\'itli the general 

 introduction of the microscope into tlie study of vein materials, how- 

 cA'er. 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 increas- 

 ingly 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 downw^ard 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 inclosing rock, a de- 

 scensionist theory, but of essentially modern type. 



In 1898 appeared the w^ell-known paper, '' The (lenesis of Ore 

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

 science at the School of Mines in Piibram, Bohemia. Although 

 Posepny's views were b}^ no means universally accepted by geologists, 

 especially in America, all agreed that his work constituted a most 

 valuable (•ontril)ution to the science by its clear definitions of the 

 questions involved and their masterly scientific discussion. The 

 great majority of ore deposits Posepny consideriMJ to be of later 

 origin than the inclosing rocks, even those that are found in stratified 

 rocks in ai)parent conformity with the bedding. Further, that they 

 have been de})()sited l)y pi-e('ij)itation from waters of the deep circu- 

 lation below the ground-watci' le\'el. The grouiul watei" he conceiv'cd 

 descends by capillarity thi'ough rock interstices over large areas, to 

 rise again at a few points tlu'oiigli o])en channels under the influence 

 of heat. It dei'ives its mineral matter from the barysj)here, or deep 

 region, where the rocks are richer in metallic minerals than near the 

 surface, and subsecjuently deposits them in oi:)en spacers as it ascends. 

 These spaces are either spaces of discission (rock fractures) or spaces 

 of solution, the latter sonielimes being formed by ascending lliermal 

 Avaters, even where no previous crack exists. 



