132 
IDDINGS. 
to have been developed, two varieties sometimes occurring 
in one rock. Nevertheless there are localities where one or 
the other is almost the universal occurrence. In groups of 
rocks with pale green augite it will be found on close inves¬ 
tigation that the tone of green is different in different dis¬ 
tricts. 
Within narrower limits peculiar qualities of minerals dis¬ 
tinguish parts of a series of rocks at one center of eruption. 
Thus the hornblendes of the intrusive porphyrites and 
andesites at Electric peak and Sepulchre mountain possess a 
grayish tone, which varies in intensity with the rocks, but 
is quite characteristic. The color of hornblende, however, 
within a large series of igneous rocks in one district is most 
variable, ranging from green in the granular rocks to green¬ 
ish brown and reddish brown in the basic lavas. 
The color of mica when fresh is often quite distinctive of 
different groups of rocks. The particular tones of color of 
minerals in such groups of rocks is more easily recognized 
by the eye than described. 
The association of minerals in different localities is even 
more characteristic. Some of the more noticeable of these 
may be mentioned. 4 
In many groups of volcanic rocks, including those of the 
Great Basin, the Pacific coast of North America, Central Amer¬ 
ica, parts of the Andes, Hungary, and in Great Britain, hyper- 
sthene is an essential constituent with a very wide range, ex¬ 
tending in many cases from certain forms of basalt through 
the andesites and dacites to many forms of rhyolite. It ac¬ 
companies augite in nearly every instance and may be said to 
be almost universally present when the rocks are fresh, though 
more abundant in some cases than others. It continues to 
accompany augite in the granular equivalents of these rocks 
to a certain extent and is found in the porphyrites, diorites, 
and gabbros of the Yellowstone Park region. On the other 
hand, there are groups of basalts, andesites, and dacites in 
which it is as noticeably absent. These rocks are associated 
with trachytes, phonolites, and allied rocks. Thus it is not 
