Lawson on diabase dykes of the 'Rainy lake region. 207 
transversely schists which have a strike of N. E. to E. N. E. 
The specimen taken from the middle of the dyke has the char- 
acters of a uralitic quartz diabase. The feldspar as a rule is 
remarkably fresh and occurs in the usual lath-shaped tv^'inned 
crystals of plagioclase. The crystals are commonly observed to 
be cracked transversely and the cracks filled with*a brownish 
yellow material which shows aggregate polarization. The 
augite occurs more commonly in polysomatic masses than 
in simple individuals. The 
magnetite is often surround- 
ed by rims of secondary 
brown mica. The quartz 
is apparently original and 
has numerous inclusions of 
an opaque granular charac- 
ter together with fluid inclu- 
sions with dancing bubbles, 
gas pores with black borders 
and glass inclusions oval and 
circular. 
On the south shore of the 
North-west bay of Rainy 
lake, a similar dyke cuts 
both the biotite gneiss of the 
region and the red granite 
which is intrusive through it. It is a uralitic quartz diabase. 
The feldspar is in rather stout crystals in the coarser grained 
part of the dyke, though usually lath-shaped. It is much de- 
composed and is partially replaced by quartz and chlorite. The 
polysomatic character of the augite is not prominent but this may 
be due to the fact that it is about half altered to hornblende and 
to chlorite. The augite individuals are often twinned and the 
cleavage traces are unusually well defined. The magnetite 
shows a tendency to peripheral arrangement around the altered 
augite indicative of its secondary origin. Quartz is present 
which is probably original besides that which is clearly second- 
ary. Apatite in long slender needles and leucoxene in irregular 
masses, are the accessory constituents. 
In the same dyke, nearer the contact where the texture is fine 
FiiJ. 1. 
Section of diabase, from Pipestone lake 
dyke, showing large polysomatic grain of 
augite in three granules of diverse orienta- 
tion a b c; d uralitic hornblende; e magne- 
tite. X 28. 
