Vol. 57.] OF THE YELLOWSTONE AND GEEAT BRITAIN. 



213 



^cavities of adjacent individuals communicate, these hollows naturally 

 become irregular ; and often the whitish material is aggregated in 

 porous or cavernous bands or clots, measuring up to '5 inch across. 



Hollow spherulites proper. — If any large slab of rock 

 forming part of the talus at Obsidian Cliff is examined, it is seen 

 that the cavities of the spherulites possess many shapes. Often they 

 exhibit a concentric disposition around a large central hollow j 

 sometimes the concentric arrangement of smaller cavities is almost 

 absent ; in a third case the vesicles are irregular and even stellate ; 

 while in a fourth they are almond-shaped and elongated in the 

 direction of flow. 



These cavities will be considered under two heads : — 

 (i) Those without definite form.^ — One example, irregularly 

 stellate, was surrounded by a mere shell of spherulitic growth in 

 which a radial structure was just discernible (fig. 1, I). The 



Fig. 1. — Cavities in the spherulites of Obsidian Cliff: natural size. 



II 



III 



spherulite it^lf was pale pinkish-white in colour, and the interior 

 of the cavity and part of the wall, as seen on a fractured surface, 

 were granular. Crystals of fayalite were embedded in and on the 

 walls. The exterior of the spherulite was slightly irregular, a small 

 blunt tongue projecting out in one place into the surrounding 

 material. This consisted partly of glass, partly of lithoidal flow- 

 bands. The stellate form of the cavity may be best conveyed by 

 imagining that a blunt wedge was thrust outwards from the cavity 

 into the yielding substance of the spherulite. In one place the 

 walls met at a right angle. 



^ See ladings, 7th Ann. Kep. U.S. Geol. Surr. (1885-86) p. 264, pi. xii. 

 figs. 1 & 5. 



