3(34 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



fectly continuous shells are often very hollow within. Isolated spherulites 

 in dense black obsidian, without the trace of cracking, are sometimes half 

 hollow, presenting a white skeleton of crystalline fibers and nodules, or 

 concentric shells which are dotted with minute pellets of tridymite. This 

 form, which is a typical lithophysa, is represented by fig. 3 of PI. XLI. 

 The shells are usually so delicate that parts of them fall out when the rock 

 is broken. On the white frosted substance of these lithophysa? rest the 

 honey-yellow crystals of fayalite, 1 which have not yet been attacked by 

 atmospheric agencies. In most other cases the porous nature of the inclos- 

 ing rock, when lithoidal, has allowed these agencies to attack the fayalite, 

 when it is usually more or less altered to an opaque metallic substance, 

 through the formation of ferric oxide. Cavities are less frequent in the 

 small blue spherulites, but occasionally the smallest spherules are white 

 and porous; and in places along the spherulitic layers, or through the black 

 glass, there are granulated bands of small cavities with a white, gray, or 

 pink coating. 



Besides the thin blue layers with spherulitic structure, the obsidian is 

 also banded by light-gray ones of a more crystalline or porcelain-like 

 nature. As these become more numerous the rock assumes a lithoidal or 

 stony appearance and grades into purplish-gray rock, delicately banded 

 with blue — a lithoidal rhyolite, or lithoidite in this case, since there are no 

 phenocrysts in it. This rock is thinly fissile in plates parallel to the band- 

 ing, which are sometimes not more than one-sixth of an inch thick (PI. 

 XLII). Occasionally the rock breaks into blocks which bear the strongest 

 resemblance to silicified wood. 



The lithoidite is completely spherulitic, but the spherulites are micro- 

 scopic and have a character somewhat different from that of those just 

 described, the megascopic ones being almost all porous or hollow. The rock 

 abounds in lithophysa? of great delicacy, and beauty, and often of great size, 

 the largest being a foot or more in diameter. An idea of their abundance 

 is given by fig. 2 of PI. XLIII, which represents a slab of lithoidite, in 

 natural size. The lithophysa? in this rock are mostly hemispherical, and 

 consist of concentric shells which arch over one another like the petals of a 

 rose (fig. 4 of PI. XLI and fig. 1 of PI. XLIII). The shallower ones pre- 

 sent small rose-like centers surrounded by thin circular shells (fig. 2 of PI. 



1 For a description of these fayalite crystals see the paper on Obsidian Cliff, already cited, p. 270. 



