452 
IDDINGS. 
terminations. They extend with uniform optical orientation 
toward the center of the spherulite. They exhibit in a few 
instances distinct cleavage parallel to the sides of the prism. 
The angle of extinction ranges from 0° to 10° or 12°, being 
usually low. The prisms are invariably optically negative, 
and are therefore orthoclase crystals elongated in the direc¬ 
tion of the clinoaxis. The high limit of the extinction 
angles, as well as the chemical composition of the rock and 
the spherulites, since there is no evidence of the presence of 
more than one species of feldspar, indicates that the ortho¬ 
clase is rich in soda, the molecular ratio of the potash to 
soda in the rock being 1 to 1. 
There is a difference between the end of the projecting 
prisms of feldspar and the part of the ray within the spher¬ 
ulite. The former is transparent and clear, without inclu¬ 
sions ; the part within the spherulite proper is clouded and 
granulated, as already stated. In some instances the granu¬ 
lation assumes a more definite character and has a radiating 
feather-like structure, which at once suggests the granophyric 
arrangement of quartz in feldspar. This is unquestionably 
its true character, although the quartz does not appear to 
affect the optical behavior of the feldspar rays. 
An examination of the microscopic granophyre groups of 
feldspar and quartz which occur in the same thin sections 
and which have been described in the article on Obsidian 
Cliff (p. 274) shows the same optical characters and feather¬ 
like structure. Such an intergrowth is represented in fig. 2, 
Plate 7. It is made up of feldspar crystals, which cross 
one another at a common point, or which radiate from a 
common center. These feldspars invariably have the axis 
of greatest elasticity, &, approximately parallel to the direc¬ 
tion of radiation. They have the same crystallographic 
orientation as the feldspar rays of the spherulites. The in- 
tergrown quartz does not alter perceptibly the optical orien¬ 
tation ; therefore it must be either so oriented as to have its 
axis of greatest elasticity more or less coincident with that 
of the feldspar, or it is not present in sufficient amount to 
