172 A. Hague — Deposition of Scorodite from 



spaces filled with a deposit of a brilliant green color. The 

 steam is strongty sulphurous and is emitted in large volumes. 

 The overflow is inconsiderable and runs towards the creek in a 

 shallow channel which, near the spring, is lined with the same 

 green coating." 



Like all other springs of the group this one issues through 

 fissures in the altered rhyolite, which, around the spring, is 

 everywhere coated with siliceous sinter or geyserite derived 

 directly from the hot waters. Deposited upon the sinter and 

 intermixed with it, occurs the green coating. It lines the basin 

 for about a foot or wherever the spray from the agitated waters 

 falls upon the sides. Analysis shows this mineral to be scoro- 

 dite, a hydrous arsenate of iron. The layers vary from a mere 

 coating up to an eighth of an inch in thickness. Frequently 

 the cavities and druses in the sinter are filled with scorodite 

 and occasionally it forms nodular masses from a quarter to 

 a half an inch in diameter. Wherever observed it occurs as 

 an amorphous deposit, and when pure, leek green in color. 

 ]Nitric and sulphuric acids apparently have no action upon the 

 mineral but it is readily attacked by hydrochloric acid. 



An analysis made by Mr. J. Edward Whitfield in the chem- 

 ical laboratory of the Geological Survey, after neglecting a 

 trace of sulphuric acid and a small amount of silica from which 

 it was impossible to free the mineral, shows a nearly pure sco- 

 rodite, closely agreeing with the theoretical composition. The 

 result of the analysis is as follows : 



Fe 2 3 --- -- ---- ---34-94 



As„O s 48-79 



H 2 16-27 



Total 100-00 



Other localities for scorodite are Chrome Springs, at the base 

 of Crater Hills, and one or two places in the JNTorris Basin. At 

 the Constant Geyser in Norris Basin the water is thrown out 

 two or three times a minute to a height varying from 10 to 20 

 feet. Around the vents the sinter deposits are finely laminated 

 and show incrustations with greenish tints. Upon breaking 

 off a piece of the sinter and examining a cross-section, thin 

 layers and irregular deposits of scorodite may be observed. 

 The scorodite is already partially altered to limcnite and the 

 sinter and pure mineral are more or less discolored by the 

 oxide of iron. It is difficult to obtain a sufficient amount for 

 analysis free from admixture of geyserite and iron oxide. A 

 specimen from the Constant Geyser carrying much silica, 

 yielded Mr. Whitfield as follows : 



