KEEWATIN AND COUCHICHING ROCKS. 227 
phyry, felsite, quartzite, etcetera, and a fine example is found at Rat Por- 
tage, the matrix in this case, however, being sericitic instead of chloritic. 
Numerous other instances are found in various parts of the region. No 
pebbles which are undoubtedly Laurentian have been reported from any 
of them, though granite boulders are found, as, for example, near Abrams 
rapids, north of lake Minnetakie. They resemble closely eruptive granites 
piercing the Keewatin in neighboring localities, and differ in appearance 
from the characteristic granite of the Laurentian. 
COUCHICHING ROCKS 
The Couchiching rocks are all formed of sand or clayey sand more or 
less metamorphosed. ‘The least changed were found by the writer on the 
shore of Rainy rivera mile below Fort Frances and at the Scramble mine 
near Rat Portage. They form thin beds of yellowish or, brownish soft 
sandstone lying between micaceous and chloritic schists. Under the 
microscope, beside grains of quartz there are particles of magnetite and 
numerous small prisms, probably of tremolite. In general, however, the 
Couchiching consists of biotite schist or gneiss, the quartz showing a 
clastic origin. Some of these schists contain sillimanite; more rarely 
they appear as thoroughly crystalline gneiss containing muscovite, micro- 
cline, etcetera, resembling the adjoining Laurentian and forming transi- 
tions toward it. 
The Couchiching includes no coarse clastics and is nowhere separated 
from the underlying Laurentian by a basal conglomerate. These rocks 
have been mapped by Lawson, Smith, and McInness as covering exten- 
Sive areas in the southern part of the region. Rocks of a similar kind 
occur on Manitou lake, near the lake of the Woods and at other points, 
but have not been separated in the mapping. 
RELATION OF KEEWATIN TO COUCHICHING 
Lawson suggests that the Shoal Lake and other conglomerates repre- 
sent the base of the Keewatin, and so indicate an unconformity between 
the Keewatin and the Couchiching ;* but the finding of many Keewatin 
pebbles in the conglomerates opposes this view. A striking evidence 
that the break represented by these conglomerates comes high up in the 
Keewatin instead of at its base just above the Couchiching is to be found 
at Shoal lake, where a few boulders of coarse-grained anorthosite found in 
the schist conglomerate are exactly like portions of a boss of anorthosite 
two miles away. As this anorthosite area contains masses and strips of 
characteristic Keewatin schist swept off during its eruption, it is evident 
* Geol, Survey of Canada, (887, p. 84, F, 
