518 Van Rom and Cook — New Occurrence of Pearceite. 



Art. XLIII. — A Nexo Occurrence of Pearceite ; by Frank E. 

 Van Horn and C. W. Cook. 



Introduction. 



In the summer of- 1908, Mr. R. B. Cochran, formerly super- 

 intendent of the Compania Metalurgica Mexicana at Sierra 

 Mojada, Coahuila, Mexico, presented the Department of Geol- 

 ogy and Mineralogy at Case School of Applied Science with seve- 

 ral specimens of silver, copper and lead minerals from the Veta 

 Rica mine at the locality mentioned above. We wish to take 

 this opportunity of thanking Mr. Cochran, both for speci- 

 mens and much useful information concerning the district.* 

 One of the minerals was recognized as polybasite, but blow- 

 pipe tests showed that it contained chiefly arsenic with little if 

 any antimony, and must, therefore, be pearceite. This in 

 itself was interesting, since the mineral has been found previ- 

 ously only at four other localities, namely : Schemnitz, Hun- 

 gary in 1833,f Arqueros, Chile, in 18794 Aspen. Colorado, in 

 1892,§ and Marysville, Montana, in 1896. [j The pearceite also 

 occurred in well-defined crystal aggregates, and likewise 

 appeared to be twinned, so that the specimens were sent to the 

 University of Michigan, where their crystallographic proper- 

 ties were investigated by the junior author. 



Geography and Local Geology. 



Sierra Mojada is the name of a town as well as that of a range 

 of mountains which perhaps more properly might be called hills. 

 The region is situated in the extreme western part of the 

 State of Coahuila, Mexico. It is reached from Escalon on the 

 Mexican Central railroad, a distance of 494 miles south of 

 El Paso, Texas. From Escalon, the Mexican Northern railroad 

 runs 78 miles northeast, and terminates at Sierra Mojada, which 

 lies in a valley about three miles wide. This valley is bounded 

 on the south by the range of hills called Sierra Mojada, and 

 on the north by other hills called Sierra Planchada. Ore was 

 first discovered in 1878, and is found for a distance of about 

 three miles along near the base of Cretaceous limestone cliffs 



* A complete description of this region and geological occurrence of the 

 ores was given at the Pittsburg meeting of the Geological Society of Amer- 

 ica, Dec. 29, 1910, in the paper entitled " The Occurrence of Silver, Copper 

 and Lead Ores at the Veta Eica Mine, Sierra Mojada, Coahuila, Mexico," by 

 Frank E. Van Horn. 



■f-H. Eose, Pogg. Ann., xxxiii, 158, 1833. 



X Domeyko, Min. 393, 1879. 



§ Penfield-Pearce, this Journal, xliv, 17, 1892. 



J Penfield, ibid., ii, 18, 1896. 



