EHYOLITE OF ELEPHANT BACK. 387 



concentrated to stout tablets of hematite, with brilliant crystal faces, some 

 of them set upon the fayalite pseudomorphs, and of later origin. In some 

 of the lithophysa 1 ' hematite occurs without pseudomorphs of fayalite. The 

 Natural Bridge is one of the best localities for the study of these hollow 

 forms of crystallization. 



NORTH AND EAST OF YELLOWSTONE LAKE. 



The spur of the plateau lying west of the outlet of Yellowstone Lake, 

 and known as the Elephant Back, is ribbed and grooved by numerous 

 drainage channels that cut deep gulches down its slopes and permit the 

 nature of the mass of lava forming it to be observed. The surface of the 

 slopes and the top of the plateau consist for the most part of glassy forms 

 of rhyolite, strongly porphyritic. Black obsidian, passing into red pumiceous 

 breccia like that south of Bridge Creek, is the prevailing rock, with occa- 

 sional areas of lithoidal rhyolite. The obsidian is markedly banded with 

 parallelly oriented feldspars and layers of spherulites, some of them red, 

 porous, and distinctly fibrous, with white pellets of tridymite, the feldspar 

 fibers radiating from the center of the spherulite, though often separated 

 from it by an open space (2066, 2068). But as the drainage channels are 

 followed from the top of the plateau downward the glassy forms become 

 less abundant, and in the deeper gulches the whole mass of the rock is 

 lithoidal and well banded (2067). ■ This relationship between the lithoidal 

 and the glassy forms of rhyolite is the usual one for the large flows; the 

 upper surface is glassy and more or less pumiceous, the lower part of the 

 mass is lithoidal, and the bottom of the flow, when exposed, is glassy for a 

 variable thickness in many cases, but not in all. 



Along the Yellowstone River 3 miles below the outlet of the lake, 

 about opposite the mouth of Thistle Creek, the rhyolite is lithoidal and 

 purplish gray, not noticeably banded, and full of phenocrysts, of which the 

 quartzes are more perfectly crystallized than in the greater number of cases 

 noticed. This characteristic becomes more pronounced in the vesicular 

 brecciated rhyolite which forms a massive exposure about 2 miles below 

 the head of Thistle Creek. In this rock well-developed double pyramids 

 of quartz, with smooth crystal faces, project into the cavities of the rock 

 and from broken surfaces. The sanidine is iridescent in blue and some- 

 times in more brilliant prismatic colors. Farther up the creek the rhyolite 



