SPHEEULITES IN OBSIDIAN. 363 



centric shells, combined with a radially fibrous structure. Their form is 

 more or less spherical, sometimes being depressed on one side, or they are 

 elongated into gourd shapes. They are frequently aggregated in botryoidal 

 and kidney-shaped groups. Their surface, where separated from the obsid- 

 ian, has a delicate velvety bloom, like that of a peach, which in rich shades 

 of brown and terra cotta contrasts strongly with the black glassy matrix. 



The larger spherulites, an inch or more in diameter, are usually lighter 

 colored, in shades of reddish gray, often having a blue center. They have 

 a more earthy texture than the small ones, and a distinct, radially fibrous 

 structure, with satiny luster. In some cases there is a granular spotted 

 appearance in the outer portion, and frequently a distinctly concentric struc- 

 ture is present, the shells being either broad and dense or of the most deli- 

 cate thinness. The surface of these spherulites is often ridged with rings 

 running parallel to the flow planes of the rock, closely resembling the surface 

 of concretions in sedimentary rocks. Their shapes vary from spheroidal 

 to flattened disks and hemispheres and irregular forms resulting from the 

 interference of spherules that have grown close together. 



The delicate banding of the rock which marks its flow structure passes 

 uninterruptedly through all these spherulites, although it is not always 

 recoo-nizable without the aid of a lens. This indicates that the spherulites 

 were formed after the lava came to rest aud when the parallel layers of 

 flow had become motionless. There is no definite order in which the 

 different kinds of spherulites crystallized. Sometimes the larger ones have 

 developed first; in other cases, the smaller ones. 



The spherulites are not all solid; the larger ones especially are more or 

 less porous, often cavernous. The large earthy ones, which appear densest, 

 have microscopic cavities between the crystals composing them. Others 

 have a saccharoidal texture and are made up of slightly adhering crystals. 

 Many of them have porous or open cavities within their mass, the periphery 

 often forming a solid crust or shell, like the rind of a cantaloupe. In some 

 cases these cavities ramify through the heart of a spherulite and are coated 

 with brilliant crystals. The porous spherulites often resemble pithy berries, 

 while the more open ones appear to have shrunk and cracked apart, like 

 the heart of an overripe watermelon. This character is illustrated in 

 figs. 1 and 5 of PI. XLI. The cavity is confined to the limits of a single 

 spherulite when this occurs isolated in the rock; detached ones with per- 



