412 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



traverses larger brown spherulites. They are also shown in bands of more 

 or less isolated spherulites within the brown spherulites. These spherulites 

 are represented megascopically by minute dots arranged in lines on the 

 surface of some obsidian. 



Spherulites similar to these, but slightly larger, occur crowded together 

 in bands or in irregular areas, in which case the rock is lusterless and 

 lithoidal, and is usually colored gray in various shades. In thin section 

 the finely spherulitic portions may be crowded with trichites in the same 

 maimer as the surrounding- glass is. These are more perfect as the spheru- 

 litic structure is more minute, and their form and arrangement in fluidal 

 lines are lost when the spherulites are coarser. In these cases they are 

 sometimes crowded out toward the margin of the spherulites, or they may 

 be accumulated in radial lines. 



The small spherulites exhibit an extremely minute granulation by 

 transmitted light, and appear brown ; but by incident light this is white, 

 evidently in consequence of the reflection of light from innumerable small 

 surfaces or cracks. In small spherulites lying isolated in, and also bor- 

 dering, areas of greater crystallization the centers of the spherulites are 

 granulated and brown, while the margins are often colorless and trans- 

 parent. In some cases the centers of the spherulites are colorless and the 

 outer zone is brown. In such spherulites the fibers of the outer zone are 

 more delicate than those at the center. In these spherulites the direction 

 of vibration of the fastest ray (axis of greatest elasticity) lies approximately 

 parallel to the direction of the radial fibers. The spherulites bordering 

 more crystalline areas in lithoidal rhyolite have sometimes continued their 

 crystallization a short distance into these areas, when they exhibit distinct 

 prismatic rays that project beyond the apparent periphery of the spherule 

 and resemble the teeth of a cogwheel. Sometimes the projecting prisms 

 assume a comparatively large size. Such forms are represented by a and b, 

 fig. 4. The projecting prisms have crystallographic boundaries and extend 

 with uniform optical orientation toward the center of the spherulite. Their 

 form and optical behavior show them to be crystals of orthoclase elongated 

 in the direction of the clinoaxis, with the direction of vibration of the fastest 

 ray nearly parallel to the sides of the prism. The extinction angle varies 

 from 0° to 12°, being usually low. The high limit of the extinction angle 

 in the clinodiagonal zone, as well as the chemical composition of the rock, 



