DIABASE DYKES OP RAINY LAKE. 



181 



polarization. 



The augite occurs more commonly in polysomatic 

 masses than in simple indi- 

 viduals. The magnetite is 

 often surrounded by rims of 

 secondary brown mica. The 

 quartz is apparently original 

 and has numerous inclusions 

 of an opaque granular char- 

 acter together with fluid inclu- 

 sions with dancing bubbles, 

 gas pores with black borders 

 and glass. inclusions oval and 

 circular. 



Fig. 1. 



On the, south shore of the 

 Noi'th - west Bay of Eainy 

 Lake, a similar dyke cuts 



Section of diabase, from Pipestone Lake dyke, 

 showing large polysomatic grain of augite in three 

 granules of diverse orientation a b c ; d uralitic both the biotite gneiss of the 

 hornblende ; e magnetite. X 28. '^ _ 



region and the red granite 



which is intrusive through it. It is an uralitic quartz diabase. 

 The feldspar is in rather stout crystals in the coarser grained 

 part of the dyke, though usually lath-shaped. It is much decom- 

 posed and is partially replaced by quartz and chlorite. The poly- 

 somatic character of the augite is not prominent but this may be- 

 due to the fact that it is about half altered to hornblende and to 

 chlorite. The augite individuals are often twinned and the cleavage 

 traces are unusually well defined. The magnetite shows a tendency 

 to peripheral arrangement around the altered augite indicative of its 

 secondary origin. Quartz is present which is probably original 

 besides that which is clearly secondary. Apatite in long slender 

 needles and leucoxene in irregular masses, are the accessory con- 

 stituents. 



In the same dyke, nearer the contact where the texture is 

 fine grained, the rock is much more uralitized, traces of augite 

 being observable only in cores of the compact green hornblende, 

 which has almost entirely replaced it. Apatite appears more abun- 

 dant, as do also the secondary quartz and chlorite. Garnet of a 

 pale yellowish color occurs sparingly. 

 3 



