362 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



until the black glass appears as thin bands in a light-gray rock, and finally 

 the whole mass is a laminated lithoidal rock. The lithoidal form of the 

 rock is not found in the thinner portions of the lava flow, but' only where it 

 has accumulated in the ancient channel. It is split into thin plates along 

 the planes of flow, owing to the differences in texture of the alternating 

 layers, which vary in degree of crystallization. This delicate lamination 

 and variability of crystallization will be referred to again, after the micro- 

 scopical characters have been described. 



Owing to the fact that the spherulitic structure, which is highly devel- 

 oped in the obsidian at this place, is typical of that which occurs in a great 

 number of other localities in the Yellowstone Park, it seems advisable to 

 describe it in considerable detail, in order to give a clear impression of this 

 very characteristic mode of crystallization. The spherulites form isolated 

 spherical bodies or groups of spheres, often so intimately intergrown as to 

 form layers in the rock. Their substance is lusterless and stony, dull 

 bluish gray and pink. They are of various sizes, the larger ones frequently 

 being hollow or porous. 



The simplest form of the megascopic spherulites is that of small dark- 

 blue spherules, about the size of a mustard seed, embedded in the black 

 obsidian. When broken, they appear lighter gray within, have a dense 

 porcelain-like texture, and exhibit slight indications of a radially fibrous 

 structure. They are usually located along fine lines of minute dots on the 

 surface of the obsidian. The small blue spherules are generally , crowded 

 together along these lines, or more properly along the planes of which these 

 lines are the traces, and which are planes of flow. Sometimes a number of 

 layers will lie close together with the thinnest possible sheet of black glass 

 between them, or they will unite to form a band a fourth of an inch thick, 

 whose surface is covered with protruding hemispheres. Occasionally groups 

 of spherules are prolonged in one direction, forming parallel ropes through 

 the black glass. 



The surface of the spherules is brown or red, and constitutes a plane 

 of weakness between the spherulite and the glass, along which the two 

 separate with ease, leaving a dull pitted surface on the obsidian. The 

 arrangement of the spherulites in the plane of flow is quite irregular, though 

 occasionally in arborescent figures. 



Spherulites about the size of peas have an agate-like banding in con- 



