loS Lawson on diabase dykes of the (Rainy lake region. 
grained, the rock is much uralitized, traces of augite being ob- 
servable only in cores of the compact green hornblende, which 
has almost entirely replaced it. Apatite appears more abun- 
dant, as do also the secondary quartz and chlorite. Garnet of a 
pale yellowish color occurs sparingly. 
At the contact the dyke rock is a compact 
aphanitic base in which can be detected minute 
porphyritic crystals. Under the microscope the 
base is seen to be made up of minute lath-shaped 
crystals of fresh plagioclase, augite 
grains, magnetite and chlorite sub- 
Fig. 2. stance. The porphyritic crystals 
Plagioclase from are lath-shaped feldspars occasion- 
diabase dyke, n u i j u • 4.1 i 
Northwest bay, ally broken and showmg the lam- 
Rainy 1., showing 11 • • , U i. 4-U 
effect of pressure ellae m some mstances bent, as the 
of one crystal a- 1 . r r • i • • i 
gainst another, result of pressure of one mdivid- 
ual against an angular part of an- 
other,and augite generally surrounded with an ir- 
regular border of secondary hornblende, which, 
in turn, has an outer girdle or wreath of granules 
of magnetite that have separated out in the pro- 
cess of uralitization as in fig. 3. 
In the south part of the Rainy lake and on 
the Rainy river a number of these dykes have been observed. 
One cuts the coarse granitoid gneiss of the river between 
Couchiching and Fort Frances on the south side of the river, 
and another crosses the river at the Manitou rapids. Neither of 
these has yet been examined microscopically. On the lake 
near the extremity of Gash point one of these dykes cuts the 
schists with a strike of N. W. and S. E. across the whole breadth 
of the point and traverses the islands on both sides of it. Here 
it is traceable on the point and on the islands for a distance of a 
mile. Three miles to the south east in the line of the strike of 
the dyke, a dyke occurs cutting the schists on the islands off the 
south shore of the lake which is probably a continuation of that 
of Gash point. From this point it is traceable for two miles 
across the islands to the main shore on the south side of Grassy 
narrows. Thus, this dyke has a length of at least six miles 
and has an extension to the north-west and south-east of the" 
Augite from dia- 
base dyke, North- 
west bay. Rainy L., 
showing marginal 
alteration to green 
compact horn- 
blende with an en- 
circling wreath of 
secondary magnet- 
ite. 
