30 Van Horn — Silver, Copper, and Cobalt Minerals. 



the region, but it seems certain that such must exist, and that 

 the erythrite has resulted from their decomposition. 



Barite. — Barite is common as a gangue mineral in granular 

 and lamellar crystalline aggregates. In thin sections, plumose 

 aggregates were observed which were associated with argentite 

 and silver. 



Gypsum. — This substance seems to be present nearly every- 

 where as a gangue mineral. It has always a tabular to scaly 

 habit, and consequently much resembles mnscovite, which, 

 however, never occurs at this locality. It is often white, but 

 in places is stained red by iron oxide. Crystal faces are present 

 in places, but are generally rough or curved. It is possible 

 that the original mineral may have been anhydrite, which has 

 been shown by Lindgren* to be a gangue mineral in some 

 localities. However, in this district it is probable that the 

 gypsum was directly precipitated by the interaction of solutions 

 containing metallic sulphates and carbonates. 



In regard to the other minerals mentioned by Chismf such 

 as pyrolusite, psilomelane, quartz, silver chloride, and lead 

 sulphate, they were not present on the specimens investigated 

 by the writer. Neither was the zinc carbonate spoken of by 

 Malcolm son.:}: However, there is little doubt that they are to 

 be found in the district. In any event there are but seven 

 minerals ever mentioned as having occurred in the camp which 

 the writer has not seen in well-defined specimens. Any other 

 species which are not included in the above list of 28 mineral 

 will have to be regarded as an addition to the mineral species 

 found in the Sierra Mojada district as far as has been observ- 

 able from existing literature. 



Geological-Mineralogical Laboratory, 



Case School of Applied Science. 



Cleveland, Ohio, September, 1912. 



*W. Lindgren: Anhydrite as a Gangue Mineral, Econ. Geol., vol. v, 

 No. 6, p. 522, 1910. 



f Op. cit. \ Op. cit. 



