WEUE IRASHC WAS VIS IOCK Sy SE ING 13 
between the diallage and plagioclase, and in small grains or par- 
ticles included within them. Much of it may be primary, but 
most of it is undoubtedly secondary. The biotite is found prin- 
cipally between magnetite and plagioclase as a reaction rim 
around the former mineral, although in some instances it appears 
to replace pyroxene. 
In many specimens of these rocks the pyroxene is quite fresh. 
In other sections the diallage has undergone a change by which 
hornblende has resulted. This is in the brownish-green indefin- 
itely outlined flakes and plates characteristic of secondary horn- 
blende. It occurs principally in the outer portions of the larger 
crystals, but sometimes it extends inward until it replaces their 
entire substance. The granulitized augite of the groundmass 
in which the porphyritic crystals are imbedded is also occasion- 
ally changed into hornblende, but only when in the near vicinity 
of the larger grains. Most of the granulitic augite is unaltered. 
Biotitic and Hornblendic Varieties —As the biotite and horn- 
blende in the granulitic rock increase in amount the pyroxene 
decreases until the first two minerals begin to affect materially the 
character of the rock and to give it a distinctive aspect in the thin 
section. Sometimes the biotite occurs unaccompanied by horn- 
blende, though this is rarely the case. It usually occurs together 
with the amphibole, with the latter in no inconsiderable quantity. 
The biotite is in two forms, either as small plates with the 
general outlines of small granulitic augite grains, or in larger 
very irregular flakes, many of which are so closely associated 
with magnetite that they must be considered as having originated 
through its alteration. The smaller plates of the mica and much 
of the material of the larger ones are very probably alteration 
products of pyroxene. The former have the same shapes as the 
granulitic grains of this mineral, while the latter often seem to 
grade insensibly into comparatively fresh augite substance. 
Some of the plates surround cores of fresh diallage from which 
they are separated by a sharp line and some occur as well-defined 
plates scattered indiscriminately among the components of a 
perfectly fresh rock. These are probably original. 
