C URRENT PRE- CA MBRIA N LITER A TURE 20 3 



The Couchiching rocks are all formed of clay sand, more or less 

 metamorphosed ; in general they are biotite-schists or gneisses, the 

 quartz showing a clastic origin. The Couchiching passes up by transi- 

 tion into the Keewatin, and there is no reason why the two together 

 should not be classed as Huronian. 



Following Lawson's estimate, the Keewatin and Couchiching series 

 together sum up 50,000 feet in thickness. 



The term Laurentian is employed, as Lawson and other Canadian 

 geologists are accustomed to employ it — in a petrographical and 

 structural sense — for crystalline gneisses and granites underlying the 

 Huronian, although it is evident that these rocks have consolidated at 

 a time later than the Huronian. 



As described by Lawson, the Laurentian (Lower Archean) " occurs 

 in large isolated central areas, more or less completely surrounded by 

 the schists of the upper Archean, the encircling belts anastomosing 

 and forming a continuous mesh work." It consists chiefly of a coarse 

 reddish, often porphyritic rock, usually granite in the central part of 

 the area, but showing a foliation, generally parallel to the periphery, 

 where it comes in contact with the Huronian. 



Throughout the region the Laurentian is in eruptive contact with 

 the Huronian, and nowhere is a basal conglomerate of the Huronian 

 found. Near the contact with the Huronian, strips and fragments of 

 the Huronian are embedded in the gneiss ; also dikes of granite, peg- 

 matite or felsite generally run from the gneiss into the Huronian. The 

 larger areas of gneiss and granite are evidently batholites. Some of the 

 smaller granite bosses may be stumps of old volcanoes. The Huronian 

 schists usually dip rather steeply away from the gneiss, at an angle 

 seldom less than 45 °. Finer-grained granites cut both the Keewatin 

 and the Laurentian. However, it is not easy to say whether a given 

 granite is Laurentian or a later granite. 



It is believed that as a result of the piling up of a thickness of eight 

 or ten miles of sediments and eruptive materials, represented by the 

 Keewatin and Couchiching rocks, the slowly rising isogeotherms soft- 

 ened or fused the foundation, which rose into domes, the inner parts 

 solidifying as granite, and the outer, more viscid portions having their 

 constituents dragged into rough parallelism with the adjoining solid 

 rocks and forming gneiss. 



As the Huronian rocks south of Lake Superior and in New Bruns- 

 wick are described by Van Hise and Dawson as presenting basal con- 



