368 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



It forms the long flat-topped spurs extending out from the base of these 

 mountains, and passes under the Pleistocene valley of the Madison River. 

 It carries on its surface a few remnants of a once extensive basalt sheet, 

 and in places has been eroded down to the underlying rocks, which are 

 mostly gneiss. Throughout most .of this area the rhyolite is lithoidal, 

 reddish purple, and slightly porphyritic. Phenocrysts are not very 

 abundant in the rock southwest of Mount Holmes (1816) and in that found 

 in the vicinity of Gneiss Creek and its northeast branch (1818 to 1820). 

 At its extreme northwestern end, north of Fan Creek, the rhyolite overlies 

 a conglomerate of andesitic fragments, and is exposed in a cliff 200 feet high, 

 the lower portion being dense and lithoidal, the upper part containing litho- 

 physpe. Small lithophysa? are also found filling' the dark-purple rhyolite on 

 the summit of the ridge to the west. 



At the base of the west slope of the plateau south of Cougar Creek 

 the lava is glassy black obsidian, closely resembling that of Obsidian Cliff, 

 except that it carries a few small phenocrysts. It is spherulitic, with small 

 blue spherulites (1864) and small lithophysae containing- quartz, tridymite, 

 and fayalite. Part of the obsidian is filled with small lithophysse, which 

 are mostly hollow, gaping spherulites, with very distinct delicate prisms, 

 which radiate from what was once the center of the pasty spherule, and 

 consequently appear to have been formed prior to the cracking and gaping 

 of the spherule. The sides of the hollows are dotted with brilliant pellets 

 of tridymite. These very hollow lithophysse, some of which exhibit a 

 tendency toward concentric shells, existed as hollow bodies with very slight 

 but rigid shells before the surrounding magma solidified, for a number of 

 them have been crushed in such a manner as to prove that the thin shells 

 were rigid and that the matrix was very viscous and the pressure not very 

 great; for the glass has not forced its way into the cavities, and in one case 

 did not fill up the space made by the cracked shell. 



At the west end of Madison Canyon the river cuts through lithoidal 

 rhyolite, which forms the western foothills of the plateau south of the 

 river. It is light colored, with lithophysse and spherulites, and in one place 

 is full of irregular cavities which are coated with crystals of quartz and 

 hematite. The quartz is prismatic, with the unit rhombohedrons and a 

 steeper one less strongly developed, but j)erfect when present. The hema- 

 tite is in thin tablets with crystal faces, and is usually twinned (1867). 



