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fine-grained gneisses. Their character throughout the 

 area is remarkably uniform. They are believed to repre- 

 sent in a highly anamorphosed condition the old sediment- 

 ary crust through which the Keewatin igneous rocks were 

 erupted and poured out as flows. 



Keewatin. — The basic types, gabbro, diabase, diorite, 

 basalt, tuff, and their schistose equivalents predominate 

 in this district. They are metamorphosed to variable 

 degrees, some of the gabbros and diorites being quite un- 

 altered. 



Hornblende Gabbro. — Thick sill-like masses of a 

 rather schistose gabbro occur in association with the Kee- 

 watin series. The predominant femic mineral is horn- 

 blende, but large idiomorphic crystals of basic feldspar are 

 abundant. Contacts with the Keewatin are exposed at a 

 number of localities, where the gabbro is seen to be clearly 

 intrusive into the Keewatin. The fact that it is wholly 

 confined to the Keewatin area suggests that the intrusion 

 is a sill or sills injected into the Keewatin prior to its 

 deformation and metamorphism. 



Algoman. — This group consists of medium-grained 

 biotite granite and granite gneiss, with less important areas 

 of mica syenite, and mica syenite gneiss, which have in 

 this district developed a remarkably basic facies. On 

 account of the warm flesh to pink colour of the granite, of 

 the fresh biotite it contains and of the marked contrast 

 in the general appearance of the rock, with the highly 

 sheared Laurentian granites of Rice bay and other localities, 

 Dr. Lawson identifies this group with the Algoman. 



Structure of the Area. — The stratigraphical relations 

 of the Coutchiching, Keewatin and Algoman groups are 

 well exposed on the shores of the lake, and may be seen at 

 different points along the route. An examination of the 

 strikes and dips of the Coutchiching as platted at different 

 points on the map shows that the formation is in the shape 

 of a large symmetrical anticline with the axis striking N.E.- 

 S.W. Locally around bosses of granite, the anticline 

 approaches a dome in form, with the planes of the Cout- 

 chiching dipping on all sides away from the granite. The 

 gradual passage of nearly flat-lying beds along the anti- 

 clinal axis to more steeply dipping ones vanishing under the 

 Keewatin belts on the east and west shores, may be easily 

 traced. No unconformity, other than an abrupt tran- 

 sition from the distinctly igneous rocks of the Keewatin 



