CONSTITUTION AND ORIGIN OF SPHERULITES. 423 
centers on the surface of the cavity, but in the outer zone 
they have all come into position normal to the circumference. 
These needles branch or fork at low angles in the manner 
well illustrated by Iddings and others, and which is quite 
common in large spherulitic growths. Between the feldspar 
needles is the silica, often in amorphous form, probably opal, 
and colored light yellowish brown by ferritic pigment. But 
a portion of the silica has usually become anhydrous and is 
developed in minute balls of tridymite, scattered wholly ir¬ 
regularly through the mass. The ferritic matter is excluded 
from these tridymite aggregates and is gathered in the 
amorphous material about them, or, if the silica is all devel¬ 
oped in such aggregates, the pigment becomes concentrated 
in larger particles and the whole spherulite is thereby clari¬ 
fied. The feldspar needles are naturally most plainly seen 
when the silica is thus developed. In some cases irregular 
quartz grains develop instead of tridymite, and then the op¬ 
tical action of the feldspar needles is more or less obscured 
by the mineral of stronger double refraction. It is charac¬ 
teristic of hollow spherulites that magnetite should be formed 
in distinct grains scattered evenly through the fibrous mass 
and in no way accentuating the radial structure. 
The second spherulitic growth, or the first where the hol¬ 
low ones are missing, has been termed the trichitic type. In 
form these growths are most variable, the tendency being to 
develop in bushy shapes or peculiarly lobed modifications of 
the round form. This variability is due to the fact that the 
conditions attending the growth caused the extremely deli¬ 
cate feldspar fibers to curve and branch to an unusual de¬ 
gree, and this influence was so strong that the magnetic iron 
assumed a trichitic form, and, starting from the same centers 
with the feldspars, branched and curved about with all the 
eccentricities of the silicate mineral. While in actually 
small amounts, the delicate black lines of these trichites are, 
when.fresh, the most striking element of these spherulites. 
The feldspars are very thin indeed, averaging only 0.004 ram 
in diameter. In trichitic spherulites the silica is developed 
as in the hollow ones, though less frequently as tridymite. 
