LITHOPHYS^ AT OBSIDIAN CLIFF. 365 



XLIII). The disks are sometimes oval, and are sometimes composed of 

 several sets of shells, which have been started from centers near together 

 and have developed in sections, giving a scalloped form to the curves. 

 Others are eccentric or send out long curving arms, as fig. 2 of PI. XLI. 



The concentric shells are generally very thin, and often so close 

 together that in one instance fifty occur within a radius of 2 inches. They 

 are very fragile and crumble under the touch, being made up of small and 

 slightly adhering crystals of brilliant quartz and orthoclase. In the litho- 

 physa represented on natural scale by fig. 1 of PL XLIII the rose-like 

 center is surrounded by delicate shells. The outer portions to the right 

 and left are somewhat massive, though finely porous and crystalline, and 

 are traversed by well-marked shrinkage cracks, which gape open from the 

 base of the rock to which the lithophysa adhered, and clearly indicate the 

 contraction of the massive portion. In the cavities between some layers 

 of the laminated lithoidite there are small tabular crystals of sanidine, with 

 blue iridescence. They are Manebach twins, the basal pinacoid being 

 the tabular plane. They contain soda and potash in equal molecular 

 proportions. 1 



In recapitulation, then, this rhyolitic lava is a flow about 100 feet 

 thick, except where it has piled up in a small valley. It is glassy, except 

 the lithoidal portion in the valley, and is free from phenocrysts The 

 obsidian is dense in the lower part of the sheet and carries numerous 

 spherulites. Large vesicles occur in the upper portion, and toward the 

 surface of the flow the spherulites disappear and the glass becomes filled 

 with gas cavities and passes up into pumice. The lithoidal portion is filled 

 with lithophysee and has numerous porous layers. These characteristics 

 repeat themselves in the rhyolite in various parts of the Park. 



The columnar structure which is so well developed in this obsidian is 

 also observed in the rhyolite in many other places in this region. It occurs 

 in the porphyritic lithoidal rhyolite on the west side of Obsidian Creek, 

 directly opposite the cliff, and is shown in the accompanying illustration 

 (PI. XLIV). These columns are short and stout, about 15 feet high and 2 

 or 3 feet broad. They are traversed by pronounced jointing, which splits 

 the rock into plates parallel to the planes of flow. From the inclination of 

 these it would appear that the original surface of the lava sloped northward 



1 Obsidian Cliff, loc. cit., p. 267. 



