SALMON — ON THE FORMATION OF ORE-VEINS. 



367 



must have exercised an influence upon it. But if, on tlie extingmisli- 

 ing of the plutonic acti^-it}" of tlie whole district the mean tempera- 

 ture of each level continued, in consequence of the cooling, to 

 decrease gradually while these processes of dissolving and vein- 

 depositing still continued, it is evident that the zones of unequal 

 deposition must descend deeper, and become mixed together pro- 

 miscuously. Since we have found that the original inequality of 

 temperature and the variations existing in the neighbom^ing rocks 

 might give rise to a zone-like distribution of the materials, so the 

 result would necessarily become much more complicated by the 

 alteration of one of these conditions — temperature. The same 

 materials which were deposited at the beginning in the upper levels, 

 next the walls of the fissures, as outer bands, would by the con- 

 tinued cooling be repeated in deeper regions, in the middle of the 

 vein. But the consequences would become yet more complicated, if 

 we must assume that in a vein-district all the fissures were not 

 opened at the same time, but by degrees, at successive periods, and 

 after the process of filling up had already begun ; and further, that 

 of those formed at the same time, some became quite filled up before 

 others. As there results from a gTadual depression of the tempera- 

 ture of the whole vein-region a kind of series and succession of suc- 

 cessively deposited minerals, so it would happen from the different 

 periods of fissure-formation that certain fissures might only contain 

 the earliest mineral-deposits, others only the latest, while some 

 vfcald ha^'e a very extended succession. Thus, and no otherwise, 

 can the so-called vein-formations of distinct districts have originated. 

 They are nothing else than the products of unequal stadii of cooling 

 of one and the same eruptive or vein-forming process. The frequent 

 parallelism and grouping together of veins of similar formation is 

 very easily explained by the circumstance that earthquakes generally 

 open more or less parallel fissure-groups. We need only consider 

 such a vein-region as a region in which, in the course of a thousand 

 years, many fissure-opening disturbances have followed upon each 

 other. 



If these so-called vein-formations sometimes show great analogies 

 of mineralogical composition and of successive age in very remote 

 regions, we need not after this consider them in any way as contem- 



