2 PROCEEDINGS OF THE NATIONAL. MUSEUM vol. G8 



OBSIDIAN CLIFF 



The Obsidian Cliff, Avhich is so well known for the beauty and 

 delicacy of its lithophysae and the minerals found in them, flanks the 

 road leading from Mammoth Hot Springs to the Norris Geyser 

 Basin, on the west. Iddings describes it as follows : '' 



Obsidian Cliff is at the northern end of Beaver Lake, in the Yellowstone 

 National Park, about 11 miles south of Mammoth Hot Springs. It forms the 

 eastern wall of a narrow cut in the plateau country through which Obsidian 

 Creek flows at an elevation of 7,400 feet. The cliff extends for half a mile, 

 rising from 150 to 200 feet above the creek and falling away gradually to the 

 north ; the upper half is a vertical face of rock, the lower portion a talus 

 slope of the same material. 



******* 



The cliff pre.sents a partial section of a surface flow of obsidian which 

 poured down an ancient slope of rhyolite from the plateau lying to the 

 east. * * * 



The exact point at which this obsidian broke through the older I'ocks and 

 reached the surface has not yet been discovered ; Imt that forming Obsidian 

 Cliff has evidently flowed down from the high plateau in a northwest direction 

 into a preexisting valley, the planes of flow in the lava clearly indicating 

 that it has crept down the slope back of Obsidian Clift" and accumulated in 

 the bottom of a channel between rhyolite hills. 



The detailed petrography of this occurrence has been given by 

 Iddings, so it is not my purpose to discuss that phase of the subject ; 

 the mineralogy shows some interesting features touched upon but 

 casually by him. and will be considered in this paper. 



GENERAL DESCRIPTION OF THE ROCKS OF THE AREA 



The rhyolitic rocks of the Yello\vstone National Park are wholly 

 extrusive. They are superficial flows in the form of nearly hori- 

 zontal sheets which vary but little in chemical composition but 

 show a multiformity of physical aspects. Lithoidal and glassy facies 

 are both abundant. The color varies as much as the physical fabric 

 and ranges from a very light gray through shades of gray and brown 

 to black, while some phases are red or mottled, as the variety 

 called marekanite. The rocks are usually porphyritic and, even in 

 some of the hyaline facies, phenociysts of feldspar or, more rarely, 

 quartz are present. In some areas vesicular or lithophysal rocks are 

 abundant and it is often in these that a variety of minerals of un- 

 usual interest is found. These reach their ma.ximum development 

 in the flow maldng up the Obsidian Cliff and here the relation.ship 

 of the minerals to one another and to their matrix is most apparent. 



or.siDiAX 



The obsidian of Obsidian Cliff is a dark, glassy rock with con- 

 choidal fracture, entirely typical of all other obsidians in appear- 



".J. p. Iddings, 7th Ann. Rep. U. S. Gool. Surv., 1888, p. 255. 



