DIABASE DYKES OF RAINY LAKE. 177 



with dancing bubbles, forms a considerable pi-oportion of the mineral 

 constituents of the rock and is characterised by having a common 

 oi'ientation for isolated sections over a wide area of the microscopic 

 field, as in the micropegmatite structure. A few colorless garnets 

 are also pi-esent. The rock, such being its characters, may be classed 

 as a ui-alitic quartz diabase. 



At 20 feet from the contact the rock is ver^' similar to that at 60 

 feet but is much less coarse in texture. It differs from the latter 

 in mineralogical composition in the fact that there is present an 

 abundance of white , or colorless garnets, all perfectly isotropic. 

 They have a well defined border indicative of a high index of re- 

 fraction and a perceptibly rough surface. Their shape is for the 

 most part rounded, or, when lectilinear outlines are observable, they 

 are hexagonal sections of the rhombic dodecahedron. The larger 

 grains have a curved parting which may be demarkation lines 

 between different i 'dividuals. The treatment of the slide with 

 hydrochloric acid cold or hot, leaves them unaffected. The occur- 

 rence of garnets in basic dykes is by no means unique. They 

 are however regarded as a product of contact metamorphism within 

 the dyke. Speaking of the " Iron District of Lake Superior," 

 Wadsworth says, " Most of the "diorites" (uralitic diabases) here 

 (at Republic Mt.) contain garnets, this mineral being found principally 

 along the edge of the intrusion vvhile the centre was nearly if not 

 entirely free from it. Theschist in like manner near the "diorite " 

 frequently contains garnets both rocks appearing to have mutually 

 reacted upon each other." * The garnets in the Jack Fish Lake 

 dyke do not appear to be a product of contact metamorphism since 

 they are found in the middle of the dyke and very much more 

 abundantly at 20 feet from the contact than at 6 feet from it, or 

 immediately at the contact, where their presence has not been 

 detected. Beyond the abundance of garnets, the dyke at 20 feet has 

 the same chai*acters as at 60 feet. The polysomatic structure of 

 the augite is pronounced. 



At 6 feet from the contact the rock is fine grained and the ophitic 

 sti-ucture of typical diabase is much more charactei-istically developed 

 than in the coarser grained parts of the dyke. In this part of the 



" Notes on the Geol. of the Iron and Copper Districts of Lake Superior. Bull. Mus, Comp. 

 Zool. Harvard, 1880, pp. 45, 46, 47. 



