ORE DEPOSITION. 233 
The generalization may be safely made, however, that most of the ore 
occurs throughout the district in the vicinity of the Weber shales, near the 
contact of these shales with the underlying rocks, and much of it actually 
occurs in this contact, which is formed by the Silver fault. 
PROCESS OF MINERALIZATION. 
The ore which occurs along faults and fractures extends into the 
apparently solid rock on both sides irregularly for a short distance. Micro- 
scopic study shows the process by which the metallic sulphides have 
replaced the original rock in these cases, for most of the ore is only a 
partially or even a slightly altered limestone or dolomite. In every case 
where examination was made of such ore the rock was found to be 
traversed by numerous reticulated fractures, along some of which micro- 
scopic faulting has taken place, all this show- 
ing the effects of great strain consequent upon 
the fault movement. Along these crevices the 
ores are in every case first introduced, and 
often this is the sole method of mineralization. 
Where the alteration has been greater, how- 
ever, the metallic minerals penetrate from the 
fractures into the rock on both sides. The 
solutions seem first to travel between adja- 
cent crystals of calcite or dolomite, and also 
Fia. 11.—Fossil changed to native silver. 
along the cleavage planes of these minerals, this cleavage being espe- 
cially well developed in consequence of the straining. In this way a 
still finer network is formed, which, by spreading and widening, has, 
in extreme cases, finally consolidated and formed a continuous mass of 
sulphide. There is no doubt that this is an actual process of replacement, 
the calcite or dolomite being taken up molecule by molecule and replaced 
by metallic minerals. 
A further evidence of this process. of replacement is the finding of 
fossils which are completely interbedded in the ore, or have been so changed 
as to forma part of the ore. Fig. 11 shows a mass of pure native silver, 
just as it was taken from the ore at the sampler in Aspen. In this silver 
part of a perfect fossil gasteropod is firmly embedded, and it is somewhat 
