38 Reviews — Dr. A. C. Laicson — The Rainy Lake Region. 



blende-schists and diabases form the lower, and the felsites and 

 felsitic schists the upper portions of the series, according to the 

 author. 



Now the Laurentian rocks of the district under consideration 

 occur in " central circular or oval areas." Each area is isolated at 

 the surface from its neighbours by a belt of Keewatin or Coutc- 

 hiching rocks. In most cases the breadth of this belt is very small 

 in comparison with the diameter of the Laurentian areas. The 

 author has traced out the boundaries of three of these areas, which 

 he names the Obabicon, the Sabaskong, and the Stanjikoming areas 

 respectively. He has also partially mapped out two other large 

 areas which lie to the N. and N.E. of the Stanjikoming area. To 

 give some idea of the scale on which the phenomena are developed, 

 we may mention that the Stanjikoming area measures thirty-two 

 miles from N. to S. and forty-six from E. to W. It is separated 

 from the Sabaskong area on the N.W. by a belt of Keewatin rocks 

 about three miles in width, and from the Lake Harris area on the 

 N. by a belt of the same rocks about 4^ miles in width. 



The contact of the two groups of rocks is well exposed at many 

 points, especially on the shores of the numerous lakes. It is 

 everywhere the same, and leaves no doubt as to the relative ages of 

 the rocks. The gneisses are intrusive. They send out apophyses 

 into the adjacent rocks, and contain included fragments, often in 

 immense numbers. The apophyses are sometimes in the form of 

 sheets running parallel to the schistosity or bedding of the sur- 

 rounding rocks ; sometimes in the form of veins or dykes cutting 

 across the bedding. The rocks in contact with the Laurentian are 

 not always the same ; sometimes they are the hornblende-schists 

 which form the basal members of the Keewatin series ; sometimes 

 the felsitic rocks which constitute the higher members of the 

 same series, and sometimes the biotite-schists of the Coutchiching 

 series. 



The Laurentian rocks are divided by the author into two principal 

 groups depending on the amount of quartz which they contain, and 

 termed respectively syenitic and granitic gneisses. Thus we have 

 hornblende-syenite gneiss, biotite-syenite gneiss, hornblende-granite 

 gneiss, and quartzose biotite gneiss. The constituents are ortho- 

 clase, microcline, plagioclase, quartz, biotite, pyroxene, hoi'nblende, 

 sphene, epidote, zircon, magnetite. The hornblende frequently 

 contains cores of pyroxene, and the author is inclined to regard the 

 whole of it as paramorphic after pyroxene. The general micro- 

 scopic structure is granitic, and crush phenomena are as a rule 

 entirely absent. Foliation is generally well marked, but massive 

 areas may sometimes be observed. The different varieties shade 

 into each other. 



In the Stanjikoming area the syenitic gneisses occur on the 

 borders ; the quartzose biotite-gneisses in the central area. 



One striking feature which is brought out by an extended survey 

 of the Laurentian areas is the concentric character of the foliation. 

 At the margins the strike of the foliation is parallel to the 



