and the lamimation of acid lavas. BT 
are seen to be made up of jointed fibers composed of micro- 
scopic feldspar crystals; between these fibers are scales of tridy- 
mite with scattered grains of magnetite and microlites, besides 
innumerable gas cavities. Frequently the tridymite is agere- 
gated into pellets which enclose several feldspar fibers, leaving 
quite porous spaces between. The centers of these larger 
spherulites often have the same structure as the smaller sphe- 
rulites, and the fibers of one appear to be continued into the 
other, but to have formed under somewhat different conditions, 
since the cementing mineral is quartz in one case and tridymite 
in the other. 
In the porous portions of the large spherulites, intergrown 
with the feldspar fibers and appearing to be of later crystalliza- 
tion is an occasional individual of yellow iron-olivine or fayalite. 
So that the association of minerals here developed, tridymite, 
acid feldspar and fayalite, is quite an uncommon one for an 
igneous rock. 
The spherulites are frequently so porous as to be hollow in 
places. Sometimes a large cavity is situated in one side of a 
radially fibrous spherulite. In this case the fibers project into 
the cavity and are distinetly visible without the aid of a magni- 
fying lens, the pellets of tridymite being also recognizable. 
The cavity may be near the circumference of the spherulite 
and adjoining the enclosing matrix, or lie nearer the center 
leaving a dense periphery or crust like the rind of a melon. 
This is the more usual form. Very often the central portion 
appears to have shrunken and cracked apart, the surface mark- 
ings on the opposite sides of the gaping cracks corresponding 
exactly, and showing that the walls had once been united in g 
continuous mass. Another variety has the cavities in concen- 
tric layers, with delicate partition walls between them forming 
thin concentric shells. When these have been developed in a 
lava to which there is a laminated structure, marked by layers 
of microlites and other forms of crystallization, the concentric 
shells of the hollow spherulites are traversed by thin layers of 
crystals in continuance of the lamination in the matrix. : 
The large hollow spheralites are generally hemispherical, and 
the concentric shells and cavities produce. very beautiful struc- 
tures, which when broken across present roselike centers sur- 
rounded by delicate encircling rings. These forms are espe- 
cially characteristic of the lithoidal portion of the obsidian flow 
at Obsidian Cliff and are the typical structures which have been 
called lithophyse by von Richthofen* because they appear to 
have been inflated by expanding gas, the word being derived 
* “Studien aus den ungarisch-siebenbiirgischen Trachytgebirgen.” Jahrb. d. 
k. k. geol. Reichsanst., 1860, p. 180. Also, The Natural System of Volcanic 
Rocks, San Francisco, 1868, p. 14. ; 
