646 IDAHO MINING DISTRICTS. 



The first process is often observed in the silicification of various 

 sedimentary, porous rocks, chiefly sandstones or tuff or porous igne- 

 ous rocks, such as certain trachytes and andesites. Silicification by 

 the cementation of shattered rock masses by silica is, of course, a 

 common occurrence in and near quartz veins. But silicification by 

 replacement is a less common process, and is observed chiefly in the 

 case of easily soluble rocks, such as limestone or calcareous shales, 

 when it results in fine-grained or cryptocrystalline aggregates of 

 silica. In the metasomatism of bodies of massive rocks penetrated 

 by chemically active solutions silica is formed in many ways, as by 

 the carbonatization of silicates and sericitization of the feldspars, 

 and if no open spaces are available much of this free silica will be 

 deposited within the rock, usually as fine-grained aggregates more 

 or less mixed with opal and chalcedonite. If no material were added 

 the final result of this would not, however, be a silicification, but 

 merely an increase in the total free quartz of the rock. But in case 

 the rock mass is cut by fissures it appears that most of the resulting 

 free silica is not deposited in the rock, but finds its way out in the 

 open ducts, where, if the solution is supersaturated, it will be depos- 

 ited. In fact, in the metasomatic processes in the ordinary igneous 

 rocks adjoining gold-quartz veins, late investigations have shown 

 that certain elements are added to the rock, while others, notably 

 silica, are frequently subtracted, to be carried away or deposited in 

 available open spaces. 



As for the other possible process of silicification, or a dissolving of 

 the original mineral and a deposition of silica pari passu, it occurs 

 chiefly in easily soluble minerals, such as calcite. In case of the 

 ordinary rock-forming silicates it is apparently not common. The 

 resulting silica is generally in the form of fine, cryptocrystalline 

 aggregates. Rocks silicified by either of these metasomatic processes, 

 or by a combination of both, may occur, but, so far as the writer's ex- 

 perience goes, are not often encountered as wall rocks of auriferous- 

 quartz veins. But neither of these processes can have produced the 

 massive, white, coarse-grained quartz of gold veins belonging to the 

 normal type. This quartz, which contains native gold and sulphides, 

 shows, under the microscope, a peculiar, coarsely granular structure, 

 the grains being partly bordered by crystallographic surfaces. This 

 structure could have been developed only by free crystallization in 

 open spaces. It is scarcely necessary to call attention, in addition, to 

 the frequency of comb structure, etc. , proving also the same kind of 

 origin. This does not necessarily mean that all large bodies of quartz 

 have been deposited in an open space, as large as the volume of quartz 

 now is. Repeated openings of the fissure have doubtless often taken 

 place. 



In nature the complication of the fissure veins is often great, and 

 it is clear, in fact, that it must be so, for the walls are often shattered, 



