THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 331 



motasoinatic roi)lacement of tlio inclosin<r rock, which in these cases 

 was liinest-one. Later observations 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 phiyed an important 

 part in rephiciniii; by ore portions of the country rock which, under the 

 (.1(1 views, mioht have been r('<rarded as vein filling. The interest and 

 iinj)()i-tance of this view were speedily recognized, especially by 

 American geologists and mining engineers, and while still novel, it 

 was doubtless sometimes applied without sufficient j^roof as an expla- 

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

 of the filling of cavities or interstitial spa(;es. With the general 

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

 cvn\ a comparatively sure method was provided of distinguishing 

 the i-esults of the two processes. The process of verification has in 

 this case resulted in the establishment of the importance and increas- 

 ingly wnde 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 inclosing rock, a de- 

 scensionist theory, but of essentially modern type. 



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

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

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

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

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

 valuable contribution to the ,science by its clear definitions of the 

 questions involved and their masterly scientific discussi(m. The 

 great majority of ore deposits Posepny considered to be of later 

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

 rocks in apparent conformity with the bedding. Further, that they 

 have been deposited by precipitation from waters of the deep circu- 

 lation below the ground-water leA'el. The ground water he conceived 

 descends by capillarity through rock interstices over large areas, to 

 rise again at a few points through open channels under the influence 

 of heat. It derives its mineral matter from the barysphere, or deep 

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

 surface, and subsequently deposits them in open spaces as it asc(^nds. 

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

 of solution, the latter sometimes being formed by ascending thermal 

 waters, even where no i)revious crack exists. 



