b PEOCEEDINGS OF THE NATIONAL MUSEUM vol. 68 



chancre in appearance than becoming somewhat lighter in color and 

 duller in luster. 



Although the line of demarkation between the larger lithophj^sae 

 and the glass is sharp, they can not be removed as easily as can the 

 simpler ones. The material in the larger forms is dull in appearance, 

 radiated textures are not so apparent, and the pellets of cristobalite 

 give way to bright aggregates of tridymite. "Well-defined crystals 

 of fayalite are common in this type. 



The lithoph^^sae attain their greatest complexity and beauty in 

 the completely lithoidal rock. (See pi. 2.) Perhaps the simplest 

 of the types found are sharply defined circular areas of crystalline 

 feldspar and tridymite that form the walls of flattened cavities. 

 These become more complex through an arrangement of the crys- 

 tals in concentric rings, as many as 50 of which have been noted 

 in some specimens (pi. 3). Finally, in the most complex types, the 

 growth of these ridges is somewhat similar to that of the petals of a 

 rose. 



The materials of the groundmass of the lithoidite and the litho- 

 physae are similar, but the crystals of the latter are much larger and 

 bounded by crystal faces. 



In cross section the lithophysae of the lithoidal rock are narrow, 

 lenslike bodies with the concentric rings of crystals on both top and 

 bottom. Some have a flat floor with a low-domed roof in which the 

 rings are on the floor only while the arrangement of the crystals 

 on the roof is entirely haphazard. In general structure the walls 

 are finely laminated and rather sandy with the larger crystals con- 

 fined to the inner surface of the cavity. In minor details the 

 lithophysae var}' greatly, but in general the preceding description 

 covers the main types. 



Large, more or less porous spherulites that are obviously transi- 

 tional between the stony splierulites and the lithophysae, are abun- 

 dant in the obsidian from the east bank of the Firehole River near 

 the Madison River. These are made up entirely of rodlike feldspar 

 crystals and tridymite plates with occasional intertelluric feldspar 

 and quartz phenocrysts embedded within their mass. They have a 

 rude, concentric structure and usually show large cracks resembling 

 shrinkage cracks, that are lined with loosely coherent, sandlike 

 tridymite crystals. These spherulites appear to be the result of a 

 progressing crystallizing process that started from a nucleus and 

 spread outward through the glass and engulfed some of the pheno- 

 crysts. 



ORIGIN OF THE LITHOPHYSAE 



There have been two general hypotheses proposed to account for 

 the lithophysal structure in obsidians and the materials contained 



