12 LE JOOLNAE OF AGLROLOGY: 
d 
was ina state of perfect internal equilibrium,” and then having 
been concluded, after some internal movement or strain had been 
set up within it. In the Minnesota rock the crystal of pyroxene 
was completely formed before the granulated portion was added 
to it. The crystallization of the diallage took place in two dis- 
tinct stages, first, while the rock’s material was ina state of inter- 
nal equilibrium, during which period the perfected crystal was 
formed ; and, second, at some more recent period, when a change 
in the conditions under which the rock-mass existed afforded a 
favorable opportunity for the formation of a second generation 
of augite, which took the form of granulitic grains. The crystal 
above pictured is the most perfect one in the section. Others 
no longer possess such a distinct outline. An interior nucleus 
heavily charged with magnetite or black earthy decomposition 
products is surrounded by a clearer zone of light green pyroxene 
with few inclusions of any kind, and this in turn passes into the 
granulitic material; or a fairly clear interior may be surrounded 
by a zone of decomposition substance, marking the outline of the 
original crystal, and this be enveloped by a clear zone with its 
attendant granulitic periphery. In all cases, however, it is 
plain that the pyroxene separated from the rock-mass in two 
entirely distinct stages. 
The second generation of diallage forming, with the plagio- 
clase, the granulitic groundmass in which the larger grains lie, 
is in very light green, almost colorless grains measuring from 
.03 to.I15 mm. in diameter. They are usually longer than they 
are broad, thus giving rise to short prismatic forms, on which 
occasionally crystal faces may be detected. They include small 
particles of magnetite and are themselves often enclosed in 
plagioclase. 
The last named mineral likewise appears to be in two genera- 
tions. At any rate it occurs in good-sized lath-shaped grains, 
and in smaller ones with badly defined rounded outlines. Mag- 
netite is included in both varieties in large quantity. 
Of the few remaining components magnetite and biotite are 
the most important. The former is in large irregular grains 
