DIABASE DYKES OF RAINY LAKE. 177 



with dancing bubbles, forms a considerable proportion of the mineral 

 constituents of the rock and is characterised by having a common 

 orientation for isolated sections over a wide area of the microscojiic 

 field, as in the mici-opegmatite structure. A few colorless garnets 

 are also present. The i-ock, such being its chai'acters, may be classed 

 as a uralitic quartz diabase. 



At 20 feet from the contact the rock is very similar to that at 60 

 feet bat 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 isotrojnc. 

 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 rectilinear 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 Avith 

 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 diaba.ses) here 

 (at Republic Mt.) contain garnets, this minei-al being found principally 

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

 entirely free from it. The schist 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 

 th'3y are found in the middle of the dyke and very much more 

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

 immediately at tlie contact, where their presence has not been 

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

 the same characters 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 

 btiucture of typical diabase is much moi-e characteristically 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. 

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



