Lawson on diabase dykes of the (Rainy lake region. 209 
points'observed, for a distance that is probably very much greater. 
A specimen from the central part of this dyke, proved on ex- 
amination to have the characters of a uralitic quartz diabase. The 
plagioclase occurs in long, rather stout, lath-shaped crystals, 
which are generally so cloudy as to obliterate the twinning in 
most cases. The augite occurs both in simple individuals and in 
polysomatic masses. It exhibits the usual 
marginal alteration to hornblende and 
there is besides a certain amount of chlo- 
rite. Original magnetite is frequently sur- 
rounded by a margin of secondary biotite. 
Micropegmatitic quartz is abundant. It is 
often intimately intergrown with the feld- 
spar, and as the latter is much decomposed, 
F's- ^- would seem to replace it as a partial pseu- 
JiU-Grt^^\^^vossl domorph, but apatite needles of the same 
a?e^tWm^-the oUei-Sn- '-^sp^ct as those which occur as inclusions 
SLTati'on'^^xTs. °^"'''" in feldspar, augite, and quartz, are often 
seen to be inclosed partly in a feldspar 
and partly in a quartz grain. The primary origin of the quartz 
in spite of its micropegmatitic character, is however, not beyond 
doubt. It is to be noted that were the quartz original we should 
hardly expect to find it in such close association with the feld- 
spar. The plagioclase of these rocks affords unmistakable evi- 
dence in its idiomorphic character of its having first crystallized 
from the magma. The augite crystallized next, enclosing the 
lath-shaped plagioclase; and the cpiartz, which would be the last 
to crystallize, we should expect to find separated from the plagi- 
oclase by the augite, i. c, to fill in the interstices between the 
augite. Again although single apatites are often found extend- 
ing from a quartz grain to a feldspar grain, a condition of things 
favoring the notion of a common primary origin of both the lat- 
ter minerals, yet such a 2:)henomenon is not incompatible with 
a secondary origin for the quartz, since the replacement of feld- 
spar by quartz must necessarily be a slow operation and proceed 
particle by particle. Fvirther, if the quartz were original we 
should hardly expect to find in it inclusions of crystals of the 
first generation like apatite, which would be liable to be enclosed 
for the most part in the earlier secretions like feldspar and augite, 
