58 J. P. Iddings — Fayalite in the Yellowstone Park. 



Art. IX. — On the occurrence of Fayalite in the lithophyses of 

 obsidian and rhyolite in the Yellowstone National Park ; by 

 Joseph P. Iddings, of the U. S. Geological Survey. 



The obsidian which forms the columnar cliff just north of 

 Beaver Lake, on the road from Mammoth Hot Springs to the 

 Geyser Basins, is especially rich in spherulites of various kinds, 

 and of the more or less hollow forms called lithophyses by von 

 Richthofen,* who first described those found in the rhyolites of 

 Hungary. 



These lithopyses, which vary in size from less than a quar- 

 ter of an inch to a foot in diameter, when broken open often 

 appear like spherulites whose central mass has shrunken and 

 cracked apart like the pithy center of an over-ripe watermelon, 

 or are composed of hollow, concentric shells, either hemispher- 

 ically arranged in rose-like forms, or in segments like the cham- 

 bers of an ammonite. The walls of these cavities are usually 

 coated with transparent crystals of prismatic quartz and tabu- 

 lar tridymite, both appearing to have been deposited at the 

 same time ; scattered among these are small, opaque, black 

 crystals about 2 mm and less in length. They are tabular in 

 form, apparently with orthorhombic symmetry, and have most 

 frequently a metallic luster, and sometimes a reddish color ; 

 tney are found upon examination to be coated with ferric 

 oxide, the interior of the crystals being transparent and of a 

 light yellow color. Their optical behavior between crossed 

 nicols is that of an orthorhombic mineral with higlj index of 

 refraction, closely resembling olivine. 



A chemical analysis was made by Dr. F. A. Gooch of the 

 chemical laboratory of the U. S. Geological Survey, on a small 

 amount of material, 0*24 gram, all that was at the time 

 available. Under the microscope the crystals were seen to carry 

 a small amount of adhering quartz, and to be coated with iron 

 oxide, they were readily decomposed in hot hydrochloric acid 

 with the separation of silica and yielded the following results : 



Si0 2 25-61 



A1 2 3 trace 



Fe 2 3 14-92 



FeO 51-75 



MgO 1-66 



CaO none 



Ignition none 



Insol. SiO„ 7-02 



100-96 



* P. von Ricbtbfen. " Studien aus den. ungariscb-siebenbiirgiscben Tracbyt- 

 Gebirgen." Jabrb. k. k. geol. Reicbsanstalt. Wien. 1861, xi, 153, et seq. 



