140 W. Cross—Hypersthene-andesite. 
The description so far would answer for almost any repre- 
sentative of that sub-group of the andesites usually spoken of 
as “ original” or “ normal augite-andesite,” or as “ Augit-andesit 
im engeren Sinne,” and which is found with unvarying charac- 
teristics in Hungary, Transylvania, the Andes of South Amer- 
ica, in many islands of the South Pacific, and in the Western 
United States. 
When, however, the pyroxene crystals and grains of the 
Buffalo Peaks rock are examined in polarized light, it is clear 
that a large portion of them do not belong to the monoclinic 
augite. If, in the first place, all those individuals of which the 
vertical axis seems to lie in, or nearly in, the plane of the thin 
section, be examined, it is seen that much more than half of 
them are very distinctly dichroic, and that all of these extinguish 
light parallel to the vertical axis. e others are not visibly 
dichroic, and extinction takes place at a very decided angle, 
usually approaching 40° from the vertical axis. 
Tf, on the other hand, those crystals which are apparently cut 
at right angles to the principal axis (judging from cleavage an 
outline) are tested, more than half of them are found to extin- 
guish light when the diagonals of the prism, as indicated by 
the best developed cleavage planes, coincide with the principal 
sections of the crossed Nicols. In the remainder there is @ 
very pronounced variation from this action, and one which is 
pleochroism observed in this case does not seem to differ from 
that often described for the “augite” of the andesites. It's 
a massive rock, that any large proportion of the augite crystals 
of which the prismatic development can be seen are by chance 
