A. Hague — Deposition of Scorodite, etc. 171 



Art. XX. — Notes on the Deposition of Scorodite from Arsen- 

 ical Waters in the Yellowstone National Park ; by Arnold 

 Hague, of the U. S. Geological Survey. 



Scorodite, although a comparatively rare mineral, is usually 

 found associated with arsenopyrite in several widely separated 

 parts of the world. It occurs in minute orthorhombic crystals 

 in many well-known mining regions and is frequently observed 

 coating crystals of quartz. Hermann mentions an amorphous 

 scorodite from Nertschinsk, Siberia, with nearly the theoretical 

 composition. Quite recently Professor A. H. Chester* has 

 reported its occurrence in the Horn Silver Mine, Utah, where 

 it is observed in thin crystalline crusts and amorphous layers. 

 Crystals of artificially prepared scorodite have been produced 

 by Verneuil and Bourgeoisf by submitting metallic iron to the 

 action of arsenic acid in sealed tubes at high temperatures. So 

 far as I know, however, its occurrence as a deposition from 

 thermal mineral springs has never before been noticed. 



Scorodite is found in a number of localities in the Yellow- 

 stone Park as an incrustation deposited from the waters of 

 several hot springs and geysers. The best occurrence, although 

 the locality is one difficult of access, is at the Joseph's Coat 

 Springs on Broad Creek, east of the Grand Cafion. This 

 group of springs is situated along both sides of the stream bed 

 between rhyolite ridges which rise abruptly for two or three 

 hundred feet. Solfataric action has completely decomposed 

 the rhyolite into smooth, rounded slopes of soft earthy material 

 unsurpassed in beauty of color by any other locality in the 

 Park ; orange, yellow, vermilion and white are interblended 

 in a most striking manner. A hundred narrow vents deposit 

 crystals of yellow sulphur far too delicate for transportation. 

 Added to this coloring are the deep greens, reds and yellows 

 derived from the algeous growths lining the hot water channels 

 running off from the numerous springs. Mineral and vegetable 

 colors vie with each other in brilliancy. 



On the west side, about 100 feet from the stream and 10 

 feet above the water, is situated a boiling pool which at the 

 time of my visit, although not seen in action, I regarded as an 

 active geyser. The following accurate description is taken 

 from the note-book of Mr. Walter Weed who accompanied me 

 to the locality : " The water is perfectly clear, deep blue in 

 color, sulphurous in odor, and in constant agitation, bulging 

 and boiling vigorously for about a foot above the level of the 

 pool. The basin measures 10 by 12 feet and is edged and 

 rimmed in by a curious yellow deposit with the hollows and 



* This Journal, April, 1887. f Comptes Rendus, vol. xc, 1880. 



