SPPIERULITIC CRYSTALLIZATION. 
451 
ment when the sphernlites are more developed, sometimes 
being crowded out toward the margin of the sphernlites and 
aggregated in opaque lines between the spherulite individ¬ 
uals. The finely spherulitic parts of the rock-section also 
exhibit an extremely minute granulation by transmitted 
light and appear brown; but by incident light this granu¬ 
lated portion is white, evidently in consequence of the re¬ 
flection of the light from innumerable small surfaces or 
cracks. In the small sphernlites that lie isolated in and also 
bordering on the more crystalline portions of the rock the 
centers of the sphernlites are granulated and brown while 
the margins are often colorless and transparent. In some 
cases the centers of the sphernlites are colorless, and the 
brown color is confined to an outer zone. In such spheru- 
lites the fibers of the outer zone are more delicate than those 
of the central portion, showing a lower degree of crystalliza¬ 
tion. 
These small sphernlites when investigated with the quartz 
plate prove to be optically negative—that is, the axis of 
greatest elasticity, ft, lies approximately parallel to the direc¬ 
tion of the radial fibers. This is also true of the most minute 
colorless spherulites which occur in the glass of the obsidian 
and are represented by fig. 1, Plate 7. In the lithoidite 
the spherulites have formed in juxtaposition, so that they 
adjoin one another with more or less polygonal boundaries. 
Occasionally there is a small space between several spheru¬ 
lites where the magma has crystallized differently. These 
spaces may attain a considerable size, relatively, or may 
constitute layers of the rock. The spherulites bordering 
such spaces frequently continue their crystallization a short 
distance into them, and exhibit distinct prismatic rays that 
project beyond the apparent periphery of the spherule, and 
resemble the teeth of a cog-wheel. Sometimes the projecting 
rays assume a comparatively great size. These forms are illus¬ 
trated by figs. 3 and 5, Plate 7. In such cases the mineral 
character of the rays is clearly determinable. The project¬ 
ing rays are prisms with parallel sides and crystallographic 
