358 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



persistent and perfectly parallel to one another and to the plane of contact 

 of the overlying massive rhyolite. This friable portion passes upward by 

 insensible gradations into dark-purple dense rock with abundant pheno- 

 crysts, almost indistinguishable from the overlying nick, from which, 

 however, it is separated by a thin layer of black perlitic glass (1769) less 

 than one-fourth of an inch thick. A similar deposit of dust underlies the 

 rhyolitic sheet in the mountain north of Terrace Mountain, where it has 

 been indurated in the same manner. 



The rhyolite sheet of Mount Everts extends over the whole southern 

 half of the top of the mountain and down the slopes of the southeastern 

 spurs to the valley, and overlies a thin sheet of vesicular basalt which is 

 exposed in a number of places. It is found near the top of the plateau wall 

 south of Lava Creek, where it is about 150 feet thick, and extends south 

 into the great plateau. In the neighborhood of Osprey Falls, east of 

 Bunsen Peak, the same rhyolite sheet is exposed in the cliff near the falls. 

 Beneath, it is a deposit of light-gray rhyolitic dust, which grades upward 

 into more compact rock and then into dark glassy rock immediately beneath 

 the massive rhyolite. The rhyolite sheet is from 100 to 150 feet thick and 

 overlies 8 sheets of basalt, which are superimposed nearly horizontally, 

 and it is in turn overlain by a 50-foot sheet of basalt, the whole series, 

 including the rhyolite, being finely columnar. The surfaces of the columns 

 of the rhyolite and of -those of basalt are very similar in color, owing to 

 weathering and to the lichens, so that the two rocks are scarcely distin- 

 guished from one another at a distance, the rhyolite appearing as dark as 

 the basalt. Near the falls, however, the rhyolite columns are longer and 

 straighter, and become granular at the upper ends, where they weather into 

 pinnacles. 



The rhyolite forming the cliff at the Golden Gate continues as a 

 nearly horizontal sheet northward beneath the travertine deposit on Ter- 

 race Mountain, and forms the top of the hill north of this, where it is from 150 

 to 200 feet thick. It is exposed in a bold cliff heading an amphitheater on 

 the northeast side of this mountain, and has the same characters as in the 

 cliff on Mount Everts. It is dark purple, lithoidal, and full of rather large 

 phenocrysts. _ In the top of the cliff it is in places glassy and perlitic, and 

 contains large vesicles. Beneath the sheet, as already mentioned, there is a 

 deposit of rhyolitic dust, whose upper portion is indurated like that on 



