THE TOPAZ-BEARING RHYOLITE 181 



Solid spherulites were nowhere observed, but the delicate hollow litho- 

 physse are developed in great perfection and beaut3^ Their distribution 

 is very irregular. As a general thing, they are not abundant. In fact, 

 many portions of the flow are entirely free from them. This is espe- 

 cially true where flow structure is very pronounced and the rock splits 

 up under the influence of the weather into slabs and where the rhyolite 

 presents a dense and more strictly cryptoerystalline texture. Lithophysse 

 are also not so apt to occiir in the darker colored portions of the rhyolite. 

 In other places, especially where the rhyolite is very light colored and the 

 texture more evidently crystalline, lithophysse may occiir in great abun- 

 dance — indeed, they may crowd thick upon each other. Flow structure 

 is not altogether absent in those portions of the rhyolite where litho- 

 physae abound, but it is not very noticeable and may be seen chiefly in 

 the fact that the lithophysal cavities are often flattish and strung along 

 in more or less definite lines parallel to the direction of flattening. The 

 coalescing of such flattened cavities so as to form larger flat openings is 

 also noticeable, but the solid rock between the lithophysse shows even 

 under the microscope but scant traces of a true flow structure. 



On the natural weathered surface of the rock the lithophysse are rarely 

 well preserved, but develop into more or less irregular cavities, yet when 

 freshly quarried they are often of extreme delicacy and beauty, being 

 composed of multitudes of concentric, crystal-lined shells separated from 

 each other by equally narrow spaces. They are built up almost entirely 

 of quartz crystals with a very little sanidine and are nearly white in 

 color (see plate 12, figure 1). The quartz crystals are clearly seen to 

 line both sides of each shell and are large enough to be readily recognized 

 by means of a magnifying lens or even with the naked eye. They vary in 

 size from one millimeter downward. The forms are the customary prism 

 and positive and negative rhombohedron, resembling a simple pyramid. 

 No tridymite was observed. 



These lithophysse in some respects strongly resemble and are still more 

 beautiful than those of the now classic locality in the Yellowstone 

 National Park, Obsidian cliff, which have been described with great full- 

 ness and clearness by Iddings.* They differ from these Obsidian Cliff 

 lithophysse in the greater size of the quartz crystals that line the delicate, 

 petal-like shells, in the character of the accompanying minerals, and in 

 the nature of the enveloping rock. 



*U. S. Geological Survey, 7th Annual Report, pp. 2G5-200 ; U. S. Geological Survey, 

 Monograph 32, pt. 2, 1899, p. 41G ; U. S. Geological Survey, Bulletin no. 150, pp. 153- 

 159. 



