1060 A TREATISE ON METAMORPHISM. 



Weed." The spring issues through a granite. This granite has been 

 extensively altered into sericite, kaolinite, and zeolite. The filling of the 

 vein consists of quartz, zeolite, and calcite. Weed finds that the vein 

 filling contains a very small amount of gold and silver, and also that 

 the altered granite contains a smaller amount of silver and a trace of gold. 

 The thin sections show the presence of pyrite and hematite. Thus there is 

 actual mineralization of this vein. 6 Substantially the same phenomena 

 as those at Boulder Hot Springs have been long known to exist at Steam- 

 boat Springs and Sulphur Bank. It is therefore certain that at those 

 localities, where ore material is accompanied by the minerals characteristic 

 of secondary action in the belt of cementation, the ores were deposited by 

 aqueous solutions. If this be so, it is little short of a certainty that in 

 general the ores associated with similar gangue minerals in the belt of 

 cementation are deposited from aqueous solutions. At any rate, one who 

 maintains that ores so intimately associated with the minerals mentioned 

 as to show contemporaneous deposition are not deposited by aqueous 

 solutions must furnish evidence upon which he reaches a conclusion 

 adverse to all observed facts. 



It is to be observed that this reasoning is in accord with the funda- 

 mental hypothesis of geology mentioned at the outset of this treatise, viz, 

 that when a certain set of complex forces and agents is observed to produce 

 complex phenomena, and no other combination of forces and agents has 

 been observed to cause such phenomena, the conclusion is that the phe- 

 nomena are to be referred to the forces and agents now at work. 



A third argument showing the direct connection between the recognized 

 work of underground water and deposits of ores has been fully stated by 

 Posepny. For the banded structure so characteristic of fillings of open- 

 ings in rocks, whether these contain metals in sufficient quantity to be 

 recognized as ores or not, Posepny proposes the term crustification." That 

 crustified or banded or comb veins are usually precipitates from underground 

 waters no one doubts. It has yet to be shown that regular crustification, 

 so general a phenomenon in ore deposits, is produced by any other agent 

 than aqueous solutions. 



"Weed, Walter Harvey, Mineral vein formation at Boulder Hot Springs, Montana: Twenty-first 

 Ann. Rept. "U. S. Geol. Survey, pt, 2, 1900, pp. 227-255. 

 ''Weed, cit., pp. 248-249. 

 c Posepny, F., Genesis of ore deposits: Am. Inst. Min. Eng., 2d ed., 1902, p. 12. 



