DIABASE DYKES OF KAINY LAKE. 175 



One of tlie most characteristic of tliese dykes is one that travei ses 

 the coarse gx'anitoid gneiss of the west ai m of Jacktish Lake, which 

 lies to the north-west of Rainy Lake. Its width is 135 feet and its 

 contact with the country rock is well exposed as a sliarp line. From 

 a macroscopic examination tlie gneiss does not appear to have been 

 altered perceptibly towards the contact. Specimens for microscopic 

 examination wei-e taken from different parts of the dyke, viz., at 60 

 feet, '20 feet, and 6 feet from the contact, and at the contact. At 60 

 feet from the contact, the rock is a coarse-grained mottled gray rock 

 in which dirty white feldspar and black pyroxene are the prominent 

 constituents. Under the microscope it presents the characters of a 

 coarse-grained, comparatively fresh diabase. Augite of a pale mauve 

 tinted gray colour is abundant and often occurs in masses that fill the 

 field of the microsco])e when low powers are used. Sometimes these 

 plates of augite are individual crystals. For the most part however, 

 they are not single individuals. When examined between crossed 

 nicols the plate of augite is seen at once to be resolved into an 

 intimately interlocking mosaic of irregularly shaped grains of diverse 

 optical orientation. In ordinary light the boundaries between the 

 different members of these " polysomatic "* masses of augite are 

 traceable only with difficulty and uncertainty. There is no interstitial 

 matter whatever, the different grains being as intimately associated 

 as in the case of interpenetration twins of feldspar. That they are 

 not twins is shown by the fact that there are often as many as half- 

 a-dozen grains all of different orientation thus combined in the same 

 mass. The cleavage, by its lack of continuity over the field of course 

 indicates a differenct; of orientation in different parts of it, but the 

 cleavage traces are not strongly marked, and attention is only directed 

 to the discordance of the cleavage after the polysomatic character 

 of the mass has been rendei-ed prominent by the analyser of the 

 microscope. This polysomatic structure of augite does not appear 

 to be common. Rosenbusch does not mention it in his last comi)re- 

 hensive summary of the present state of petrographical knowledge.! 

 The nearest approach to this structure that is at all well known is 

 the polysomatic character of some chondri of olivine in certain 



- Adapted from Tschermak's use of this word as applied to a similar structure in the oli^'ine 

 of certain meteorites.— V. Die Mikr. Beschaff. der meteor. Stuttgart, 1885. 

 t Mikr. Phys. der Mineralien undGesteine Stuttgait, 1880 



