448 
IDDINGS. 
The investigation of the spherulitic forms of crystallization 
already alluded to showed that there are two different kinds, 
one of which is compact and radially fibrous, the shape of 
the individual fibers not being determinable. The second 
kind is composed of jointed fibers,. sometimes branching, 
made up of microscopic feldspar crystals; between these 
fibers are tridymite scales and often gas cavities.* 
The compact and radially fibrous spherulites range in 
size from those which are a centimeter in diameter to micro¬ 
scopic ones, .05 mm in diameter. Between crossed nicols the 
larger ones exhibit numerous dark arms, a characteristic of 
the so-called pseudospherulites. The smallest ones ex¬ 
hibit a more or less well-defined dark cross, the arms of which 
alternately contract and spread, and split into branches near 
their ends when rotated, a characteristic of the so-called true 
spherulites when highly magnified. It was stated in the 
paper published in January, 1887, that “ These spherulites 
have been traced through gradations of microstructure to 
groups of granophyre feldspars of extreme minuteness, which 
appear to be composed of intergrown quartz and acid feld¬ 
spar, and enclose trichites and microlites which also occur 
in the spherulites, so that the mineral composition of the 
spherulites is most probably quartz, acid feldspar, and tri¬ 
chites of magnetite with augitic microlites. Chemical analy¬ 
ses of the spherulites and the obsidian in which they occur 
show that the two are identical, and that a spherulite is only 
a particular form of crystallization of the once molten glass.” 
The evidence on which this conclusion was based has been 
described and illustrated with considerable detail in the 
article on Obsidian Cliff, in which the writer was led to con¬ 
clude that the spherulites are composed of feldspar and 
quartz that have crystallized from the molten glass at one 
and the same time and have intergrown with each other.” 
In his work entitled “ British Petrography,” published in 
*Amer. Journ. Science. 8°. New Haven, 1887, Jan. vol. 33, pp. 36-37. 
Also fuller article by same author on Obsidian Cliff, Yellowstone National 
Park, in Seventh Annual Eeport of the U. S. Geological Survey. 8°. 
Washington, 1888, pp. 276-278. 
