lsJO E. V. Sha/nnon — Occurrence of Ilvaite m the 



City, in Owyhee Co., which occupies the extreme south- 

 western corner of the state. The mines are reached by 

 a wagon road from Jordan Valley, Oregon. According 

 to Bell, 4 South Mountain is an isolated uplift or offset 

 from the Owyhee Range and is entirely surrounded by 

 a broken lava plateau. It rises to an extreme elevation 

 of 8000 feet and is probably 10 miles long by 5 miles 

 broad. It represents an anticlinal arch of eruptive 

 granodiorite, a medium-grained, gray, granitic rock 

 which carries an excess of hornblende over biotite. This 

 uplift has evidently been broken by a fault along its 

 axis, wmich has been eroded into a deep gulch, now form- 

 ing the bed of William Creek, which practically tra- 

 verses the center of the uplift in a northwesterly direc- 

 tion. Parallel to the bed of William Creek, and climbing 

 over the highest crest of the mountain near its source, 

 there is a belt or zone of white marbleized limestone, 

 showing a blocky structure, with black lines of silica and 

 impurities. Its general strike is northwest and south- 

 east, turning sharply to the east where it crosses under 

 the highest crest of the mountain near the head of the 

 creek. It is from 100 to 300 feet wide and makes a dis- 

 tinct white line readily traceable for ten miles. It has 

 a dip to the southwest of about sixty degrees and is 

 underlain by one thousand feet of schist, succeeded by a 

 like width of gneiss, which is in turn succeeded by a wide 

 belt of granodiorite. The same succession, partly capped 

 by basalt, recurs on the opposite side of the marble band. 



Character of the Ores. — According to Lindgren, the 

 ore minerals consist of argentiferous galena with some 

 zinc-blende and copper minerals in a gangue of quartz, 

 calcite, actinolite and ilvaite. The ores contain from 20 

 to 60 per cent, lead and from 40 to 100 ounces of silver 

 per ton. "The deposits are supposed to be veins but the 

 mineral association appears to be one clearly indicating 

 contact deposits." 



Hershey, 5 more recently, gives the following notes : — 

 The ilvaite occurs in a contact-metamorphic deposit. A 

 large area of rather fine-grained granite, or granodiorite, 

 adjoins an area of highly metamorphosed sedimentary 

 rocks now largely schists, quartzites, and marble. In a 

 broad band of marble near the granite, there is a series 

 of masses of contact-metamorphic minerals including 



6 Hershey, Oscar H., personal letter, 1916. 



