38 oS. P. Lddings—Nature and origin of Lithophyse 
from ABoc, a stone, and guoa, a bubble. The term applies 
equally well to all the concentrically chambered varieties, and 
may be extended to all forms of hollow spherulites. 
The substance of the lithophysee occurring at Obsidian Cliff 
is usually light colored and distinetly crystalline with a beauti- 
ful frosted appearance. It is made up of minute crystals in 
well developed forms, which in places attain a considerable 
size, from 1 to 2™". The minerals recognized are: quartz and 
tridymite, feldspar, fayalite and magnetite. The feldspar in 
some cases has the crystal habit of adular, and in others is in 
thin tabular crystals with the simplest combination of faces, 
flattened parallel to the base. They appear to hold an anoma- 
lous mineralogical position, being soda-orthoclase in composition 
CA Ory) and having the crystallographic habit of sanidin but 
an asymmetric optical character. The fayalite was described 
in this Journal for July, 1885. It appears to be identical in 
crystal form with the mineral occurring in the lithophysee of 
the obsidian of Cerro de las Navajas which was measured by 
Gustav Rose,* and determined to be olivine in 1827. The an- 
gles and habit of the crystals figured by him are almost identi- 
cal with those obtained by Mr. 8. L. Penfield from the fayalite at 
Obsidian Cliff, and it is highly probable that the mineral deter- 
mined by Rose is also an iron olivine or fayalite. The mag- 
netite occurs as microscopic grains and crystals. 
Since the first application of the term lithophysas to these 
hollow, chambered structures, and the expression of von Richt- 
hofen’s views as to their probable mode of formation, petro- 
graphers have discussed their origin, wandering more or less 
from the position first held by von Richthofen, and it is with 
the belief that the material on which these recent observations 
have been made furnishes better ground on which to build a 
theory of the origin of lithophysze than any heretofore studied, 
that the writer has endeavored to throw some light upon so ob- 
scure a subject. 
It was von Richthofen’st opinion, expressed in the year 1860, 
that the concentric shells were produced while the rock was in 
a molten or plastic condition, by the expansion of gas bubbles 
which were successively disengaged from certain portions with- 
in the mass, in consequence of the diminution of pressure 
accompanying the eruption of the lava. The gases which 
were in large part aqueous, having most likely been absorbed, 
thereby forming a hydrated glass. He considered lithophysze 
as quite distinct from spherulites, and suggested that only care- 
ful chemical analyses would settle their true nature. 
* “Ueber den sogenannten krystallisirten Obsidian.” Pogg. Ann., 1827, Band 
10, pp. 323-326. 
¢ Loe. cit. 
