424 
CROSS. 
Trichitic spherulites do not as a rule form extensions of the 
earlier hollow variety, but spring from independent centers. 
While in spherical form they may reach a diameter of two 
or three inches, they are much more commonly developed 
in fox-tail-like arms of a few millimeters diameter. In Fig. 
1 of Plate 5 the arborescent forms between hollow spheru¬ 
lites are trichitic growths. 
Both hollow and trichitic spherulites are often surrounded 
by a zone of supplemental spherulitic growth. In this the 
force of crystallization has been weak. Probably only a 
small part of the feldspathic material crystallized out, and 
it appears in very thin delicate fibers which are prevailingly 
oriented as extensions of the needles of the interior growth. 
Often the optical action of these fibers is so slight as to 
require special care in observation, in order to detect it. 
Further individualization of minerals in these supplemental 
areas is rare. A pigment of almost unresolvable fineness 
usually produces a very even yellowish-brown coloration, 
which sharply defines the spherulite from the surrounding 
glass. Occasionally the pigment develops in distinct flakes 
which by concentration on certain planes emphasize a wavy 
concentric structure. Supplemental spherulitic zones may 
be very narrow, or they may be even wider than the diameter 
of the enclosed spherulite. Their outer boundary is a sharply 
defined curved surface. Secondary grains of quartz some¬ 
times develop from the silica in the amorphous base. 
The three spherulitic generations which have been de¬ 
scribed locally make up the entire rock. Usually, however, 
there is more or less residual space between them that may 
be occupied by glass, by a fourth radiate growth, or by the 
two together. This residual spherulitic growth exhibits a 
regular outer form only where bounded by glass or by a 
somewhat later crystallization of the same kind. The forces 
acting in this last period favored the development of larger 
individuals of feldspar, quartz, and ore than in the preced¬ 
ing groups. 
Where the first three spherulitic growths were subordinate 
in development the fourth generation was able to acquire a 
