OF CENTRAL CANADA PART V. 297 



DISTRICT OF THE UPPER LAKES. 



This district may be described in general terms as extending west 

 of Lake Temiscamingue, over the almost entire north-west portion of 

 Ontario, exclusive of the still little known Palaeozoic region around 

 James' Bay. It forms for the greater part a densely-wooded, but 

 rugged and mountainous region, broken up by numerous bodies of' 

 water, and underlaid essentially by crystalline and semi-crystalline 

 rocks of the Laurentian and Huronian series. The surface of Lake 

 Temiscamingue is 612 feet, that of Lake Huron 578 feet, and that of 

 Lake Superior 600 feet above the sea, From these levels the ground 

 rises more or ]ess abruptly to an average height of 1,000 to 1,500 

 feet, with here and there a few points of still greater elevation. The 

 rocks within its area comprise : ( 1 ), representatives of the Lauren- 

 tian and Huronian series, occupying, as remarked above, the greater 

 part of its surface ; (2), some succeeding strata of supposed Lower 

 Cambrian age j (3), many eruptive granites and trappean masses; 

 and (4), overlying Glacial and Post-Glacial superficial deposits. 



Laurentian and Huronian rocks : The Laurentian rocks of this 

 district consist for the most part of ordinary red and gray gneiss, in 

 more or less inclined and often nearly vertical beds, or presenting a 

 highly contorted lamination. These alternate with darker beds con- 

 taining hornblende, or in some cases augite, and with occasional 

 bands of crystalline limestone. Black tourmaline or schorl and 

 opaque dark red garnets are among the more commonly-occurring 

 accidental minerals of these gneissic rocks. The Huronian repre- 

 sentatives, although distinct enough in their entirety, closely resemble 

 in many cases the Laurentian rocks of the district, and cannot always 

 be readily separated from these. As a rule, however, the texture is 

 less crystalline or less granitoidal, and slaty or semi-crystalline con- 

 glomerates appear among them. Quartzites, varying in tint from 

 colorless or pale green to dark gray and black, are especially abund- 

 ant ; and many of these hold pebbles or fragments of jasper or of 

 gneiss, or pass into siliceous slates or slaty conglomerates. In some 

 cases also they are more or less feldspathic, and consist of an intimate 

 mixture of quartz and orthoclase or other feldspar. In addition to 

 these quartzites and slate conglomerates, dark green chloritic and 

 hornblendic rocks form the more characteristic representatives of the 

 Huronian series. 



