182 



PROCEEDINGS OF THE CANADIAN INSTITUTE. 



Fig. 2. 

 Plagioclasefr'm 

 diabase dyke , 

 Northwest Baj', 

 Rainy Lake, 

 sh'wingefltectof 

 pressure of one 

 crystal against 

 another. 



At the contact the dyke rock is a compact 

 aphanitic base in which can be detected minute 

 porphyritic crystals. Under the microscope the base 

 is seen to be made up of minute lath-shaped crystals 

 of fresh plagioclase augite gi-ains, 

 magnetite and chloritic substance. 

 The porphyritic crystals are lath- 

 shaped feldspars occasionallv brok- 

 en and showing the lamellae in 

 some instance bent, as the result 

 of pressure of one individual against "**"' 



an angular part o^" another, and augite gener- ^^^JlHi ,^,,,,,, 

 ally surrounded with an irregular border of dyke, Northwest Bay, 



•' . "7 Rainy Lake, shownig 



secondary hornblende, which, in turn, has an marginal alteration to 



•^ . green compact horn- 



outer girdle or wreath of granules of magnetite blende with an encir- 



" . I. .. cling wreath of second- 



that have separated out m the process of urali- ary magnetite. 



tization as in fig. 3. 



In the south part of the Rainy Lake and on the Rainy River a 



number of these dykes have been observed. One cuts the coarse 



granitoid gneiss of the river between Couchiching and Fort 



Frances on the south side of the river, and another crosses the 



river at the Manitou rapids. Neither of these have yet been 



examined microscopically. On the lake near the extremity of Gash 



Point one of these dykes cuts the schists with a strike of N. 



W. and S. E. across the whole breadth of the point and traverses 



the islands on both sides of it. Here it is traceable on the point 



and on the islands for a distance of a mile. Three miles to the 



south east in the line of the strike of the dyke, • a d3'ke occurs 



cutting the schists on the islands off the south shore of the lake 



which is probably a continuation of that of Gash Point. From this 



point it is traceable for two miles across the islands to the main 



shore on the south side of Grassy Narrows. Thus, this dyke has a 



length of at least six miles and has an extension to the north-west 



and south-east of the points observed, for a distance that is probably 



very much greater. A specimen from the central part of this dyke, 



proved on examination to have the characters of a uralitic quartz 



diabase. The ])lagioclase occurs in long, rather stout, larh-shaped 



crystals, which are generally so cloudy as to obliterate the twinning 



