DIABASE DYKES OF RAINY LAKE, 179 



At the immediate contact the dyke assumes microscopically the 

 characters of a very compact grayish black aphanitic rock in 

 which can be occasionally detected minute glistening facets 

 of porphyritic crystals. With low powers of the microscope the 

 matrix is not resolvable but appears as an uniformly yellowish to 

 greenish gray ground thickly dotted with grains of magnetite. 

 Under the higher powers this is seen to be made up, in addition to 

 magnetite, of a tine felt-work of minute lath-shaped crystals of pla- 

 gioclase imbedded in hazy, somewhat yellowish green flocculent 

 chlorite substance derived presumably from the alteiation of the 

 augite, since that mineral cannot with certainty be identified in the 

 base. The porphyritic character of this ]>art of the dyke is well 

 marked, though the imbedded crystals ai-e small. These are augite 

 in small irregular polysomatic masses, with a hazy margin or 

 fringe of greenish decomposition product, and long lath-shaped pla- 

 gioclase and occasionally stouter broken fragments. Besides these 

 there are porphyritic crystals of enstatite much more altered and 

 less plentiful than at 6 feet from the contact. Neither quartz nor 

 garnets are observable in the contact rock. 



Considering then the dyke with reference to its viriation in 

 structure and mineral composition the points of interest to be noted 

 are : The passage of the coarse grained central portions of the dyke to 

 the compact aphanitic I'ock at the contact ; the absence of porphyritic 

 strnctui-e in the middle of the dyke as contrasted with the well marked 

 development of the same as the rock becomes finer grained towards 

 the dyke walls ; the absence of the characteristic chloritic substance 

 of diabase in the centre of the dyke and its abundance towards the 

 contact ; the presence of quartz in greater quantity in the coarse 

 grained middle portions than at the sides ; the presence of garnets 

 in the coarsest parts of the dyke, their abundance in the medium 

 grained parts and their rarity or total absence in the neighborhood 

 of the contact ; the presence of the rhombic pyroxene enstatite in 

 typical idiomorphic porphyritic crystals in the fine grained parts 

 near the contact and its absence in the coarser central parts; the 

 diminution in size of the porphyritic crystals near the contact in co- 

 extension with the increasing fineness of the ground mass ; and 

 finally the " polysomatic " structure common to the augite throughout 

 the dyke. 



