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continuously along the shores. The rugged relief of this 

 northern part of the lake is in decided contrast to the region 

 south of Grande Presqu'ile, where many sandy beaches 

 and dunes and high rocky shores are uncommon. 



According to Dr. A. C. Lawson (1), to whom our geo- 

 logical knowledge of this district is chiefly due, the Pre- 

 Cambrian rocks are separable into four principal groups: 

 Keewatin ; Laurentian ; a series of granites younger than 

 the Laurentian; and Keweenawan. 



KEEWATIN. 



The oldest of these formations, the Keewatin, is divided 

 for purposes of mapping into four divisions which appear 

 to be lithologically distinct, but at times grade so 

 imperceptibly from one to another that it is well nigh 

 impossible to draw hard and fast boundaries. These are : — 

 (a) Hydromicaceous schists and nacreous schists, with 

 some associated chloritic schists and micaceous schists, 

 and including areas of altered quartz porphyry. 



(b) Clay slate, mica schist and quartzite, with some fine 

 grained gneiss. 



(c) Agglomerates and other coarse clastic rocks, all 

 more or less schistose and generally of volcanic origin. 



(d) Hornblende schist and altered trap, with some 

 chlorite schists of volcanic origin. 



In addition to the above, some bands of carbonaceous 

 schists and ferruginous dolomite and possibly some 

 serpentine are included in the Keewatin. 



Of the four principal divisions the last two are definitely 

 referred by Dr. Lawson to an irruptive origin; the first 

 is said to have been laid down by sedimentation, though 

 probably originally volcanic, and the second is assumed 

 to be of a sedimentary origin. 



Hydromica Schists, Nacreous Schists, etc. — Dr. 

 Lawson seems to consider that the members of this sub- 

 group are largely sedimentary, though originally volcanic 

 (volcanic ash beds). He recognizes quartz porphyry as 

 the original rock from which part of the series was derived. 

 The writer's study would indicate that they resulted 

 largely from the alteration of a diorite or andesite similar 

 to the more acidic portions of the ellipsoidal trap. In the 

 development of these schists, the rock passes through a 

 stage which has been called agglomerate, though this term 



