48 Rogers — Eglestonite from San Mateo County, Gal. 



Akt. VII. — Eglestonite from San Mateo County, California ; 

 by Austin F. Rogers. 



Eglestonite is an isometric mercurous oxychloride named 

 by Prof. A. J. Moses* in honor of Thos. Egleston, founder of 

 the School of Mines of Columbia University. It has hereto- 

 fore been found only at the original locality, Terlingua, Texas, 

 where it occurs with kleinite, terlinguaite, montroydite, and 

 mosesite, all recently described mercury minerals. 



The writer wishes to put on record and describe this inter- 

 esting mineral from a California locality. Some specimens 

 from a quicksilver prospect in San Mateo County, about five 

 miles west of Palo Alto, brought to my attention by Mr. W. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



rf^^Pl 



Fig. 4. 



^k 



d 



R. Nobs, a mining student at Stanford University, contained 

 cinnabar, mercury, calomel, and an unknown yellow mineral, 

 which was soon identified as one of the Terlingua oxychlorides 

 of mercury. Careful examination proved the mineral to be 

 eglestonite. No other mercury minerals except those men- 

 tioned were found. 



Occurrence. — The eglestonite occurs in a pale-brown silic- 

 eous material in serpentine. The serpentine is an irregular 

 intrusive mass about four miles long and about three-fourths 

 of a mile wide in Franciscan sandstone. This siliceous mate- 

 rial is common in all the quicksilver mines and prospects 

 of the Coast Ranges and was formed, no doubt, during the 

 serpentinization of the original peridotite. Minerals collected 

 around this prospect include dolomite, magnesite, opal, and 

 quartz, in addition to the mercury minerals. The eglestonite 

 occurs in minute crystals and crusts in seams and cavities 

 directly associated with metallic mercury and calomel. 



Crystal form. — The crystals of eglestonite are very small, 

 the largest being only about \ mm in diameter. Measurements 



* This Journal, vol. xvi, pp. 253-263, 1903. 



