1885.1 oOo fGenlh and Vom Rath. 



Contributions from the Laboratory of the University 

 OF Pennsylvania. 



No. XXIII. 



OJN^ THE ya:n^adates a:n'd iodyrite, 



From Lake Valley, Sierra Co., New Mexico. 



By F. a. Genth and Gerhard vom Eath. 



(Read before the American Philosophical Society, April 17, 1SS5.) 



At a meeting of the Amerlcaa Institute of Mining Engineers, held at 

 Washington, D. C, in 1883, the late Prof. Benj. Silliman read a paper 

 "On the Mineral Regions of Southern New Mexico," * in which he gave 

 the first fuller information about the extraordinary occurrence of rich 

 silver ores and the accompanying minerals in the mines of the so-called 

 Sierra Group, comprising the Sierra Grande, Sierra Bella and Sierra 

 Apache. 



The geological position had been determined in 1881 by Prof. E. D. 

 Cope, when he showed that the strata in which the ores occur belong to 

 the lower coal measures. 



The ores form a bed more or less interrupted, or rather, a series of huge 

 lenticular beds or pockets between the limestone strata, which show a dip 

 of about 30° towards the S. E. This dip flattens to one of only about 15^^ 

 at a depth of 180 feet. The ore bed is frequently divided by intercalations 

 of limestone strata. 



The foot wall consists of a hard, more or less siliceous, blue limestone, 

 with very few fossils, into which the ores frequently penetrate, as if by 

 alteration of the bed rock. The hanging wall is formed by a pale slaty 

 liraeston3 without siliceous inclosures, rich in fossils, but without any 

 ores. The separation between the hanging wall and the ore bed is perfect. 



The main body of the ores is formed by oxides of iron and manganese : 

 hematite, liuionite, pyrolusite, manganile, psilomelane, and wad, inter- 

 mixed with variable quantities of cerargyrite and embolite, together with 

 small quantities of native silver and highly argentiferous cerussite and 

 galeuite. At many places the upper portion of the ore bed consists of flint 

 and siliceous minerals which cover the argentiferous iron ores, and which 

 are sometimes rich in embolite; pale yellow crystals of vanadinite in 

 druses of quartz are sometimes met with. 



These facts, mostly taken from Prof. Silliman's paper, give a brief 



* Eagineering and Mining Journal, New York, Oct. 14 and 21, 1882, pp. 1Q9 and 

 206; 212, 213 and 214. 



