Vol. 4] Smith. — Upper Region of Main Walker River. 



and fissures. The thin sections of the rock of this belt show 

 more or less alteration of the rock and substitution of quartz 

 which contains liquid inclusions. Sulphide ores make some 

 appearance farther down in the prospect, and there has been 

 considerable oxidation along the fault, and more particularly 

 because it is vertical and in the bottom of a canon. A hydrous 

 copper phosphate, which the miners termed malachite, occurs in 

 the mine, and from qualitative chemical tests made by the 

 writer it appears to be libethenite: and from some goniometrical 

 measurements made by W. T. Schaller of the United States Geo- 

 logical Survey it exhibits a few new forms. Also of some interest 

 is the formation of chalcanthite since the mining' began. It 

 appeal's to have formed in consequence of the steam which was 

 introduced in diamond drilling. The remainder of the ledges 

 occur from two to six feet wide, and the ore is much oxidized 

 and leached. In some of the microscopic slides of the ores that 

 have not been mentioned a very intimate relation of epidote to 

 chalcopyrite is noticeable. The epidote occurs quite irregularly in 

 outline, and the chalcopyrite tends to inclose it. The latter 

 usually shows one or more quite distinct faces. This would seem 

 to indicate that the chalcopyrite ceased to crystallize after the 

 epidote and before the calcite or the quartz. In this connection 

 it may be well to mention some of the considerations of a petro- 

 graphical character that guided the writer in the study of the 

 ores and their occurrence. 



There appears as a corollary to many of the contributions to 

 the subject of the formation of mineral veins, with respect to 

 their structure, that relative idiomorphism and chemical phase 

 cannot be regarded as having exactly the same significance that 

 they have in the case of the igneous rocks. Their significance 

 depends upon a different set of conditions. The banded struc- 

 ture frequently seen is due to a change in the solutions while 

 the fissure is being filled; and as the filling starts on the walls 

 and extends by successive layers or bands toward the middle, 

 each mineral composing the layers is formed against a previous 

 surface, and hence must have on one side the form of that sur- 

 face. The other sides may not be freely developed on account 

 of interference due to the minerals being contemporaneously 



