W. Linclgren — Orthoclase as a gangtie mineral. 419 



silver mine, Mexico, and gave it on account of supposed 

 abnormal angles the name of valencianite. This valencianite 

 probably occurred as gangue mineral in a vein. 



From the United States orthoclase has been noted by Genth 

 as occurring in minute crystals at the Silver Hill and Steele 

 mines, N. C. G. F. Becker has recently recognized the pres- 

 ence of small grains of orthoclase in a vein quartz from Alaska. 

 It may be added that during the examination of certain aurifer- 

 ous veins at Silver Crown, Wyoming, by the author, grains of 

 microline were sometimes found in thin sections of vein quartz. 



Orthoclase is frequently mentioned from tin veins, in 

 Saxony, Bohemia and Cornwall, and while many of these 

 occurrences are beyond doubt, yet it may be pointed out that 

 owing to the peculiar structure of these veins, some feldspar 

 from the surrounding granite might in the older reports have 

 been mistaken for true gangue mineral. While orthoclase of 

 course is common on pegmatite veins, it has not as yet been 

 proved that these ever are auriferous. 



During the examination of Silver City mining district, in 

 southern Idaho, undertaken last summer for the U. S. Geo- 

 logical Survey, a vein was encountered which from various 

 standpoints proved to be most interesting. 



The Black Jack-Trade Dollar vein outcrops on Florida 

 mountain near Silver City and can be traced for about one mile. 

 The strike of the vein is easterly and westerly ; its dip is 

 nearly vertical. It cuts three formations : a normal muscovite- 

 granite forming the fundamental rock, a basalt resting on this 

 granite and a rhyolite, capping both. As the latter two rocks 

 are of Tertiary (Miocene) age the deposit is clearly of com- 

 paratively recent origin. 



The ore minerals are pyrite, chalcopyrite and argentite ; the 

 value is chiefly in silver with a smaller amount of gold. The 

 vein is a characteristic fissure vein, well defined and having a 

 thickness of from a few inches to two or three feet. In the 

 granite it is closely followed by a basaltic dike. The ordinary 

 ore consists of typical vein filling, though low grade ores con- 

 sisting of altered rhyolite appear along certain parts of the 

 vein. The gangue is of an interesting and unusual character, 

 consisting of quartz and orthoclase, in varying proportions. In 

 general, quartz predominates, but at many places the two 

 minerals occur in equal proportions or even with prevailing 

 orthoclase. 



The first and most common occurrence of the orthoclase is 

 as large, irregular milkwhite grains, intergrown with vein 

 quartz ; it contains inclusions of pyrite and argentite and some- 

 times thin lamellae of pyrite deposited parallel to the best 

 cleavage plane. The grains appear under the microscope as 



