406 THE VERMILION IRON-BEARING DISTRICT. 



Microscopic characters. — In exceptional cases in the Yerj coarse-grained 

 rocks the texture is almost granular. More commonly we find an ophitic 

 texture imperfectly developed, with the feldspar occurring in very irregu- 

 larly bounded but in general lath-shaped fdrms, and the augite in more 

 rounded grain-like forms than is common. The normal ophitic texture 

 is very commonly developed in these rocks, and this grades over into 

 the intersertal texture which is especially well developed in the fine- 

 grained border facies. The fact should be emphasized that while the 

 ophitic texture is the one which is most commonly developed in these rocks, 

 there is occasionally observed an imperfectly ophitic texture grading into 

 a more or less granular texture, and this will be referred to later as 

 evidence in faA^or of the close relationship of the rocks of these sills with 

 the gabbro. 



Constituents. — The rocks are very fresh, and the constituents of them 

 are, in order of prominence, augite, feldspar, titaniferous magnetite, then 

 brownish-green hornblende, and some brown biotite. The biotite is occa- 

 sionally present in sufficient quantity to warrant our speaking of the rocks 

 as mica-dolerites. The only secondary mineral observed is a light-g'reen 

 amphibole derived from the uralitization of the augite. The rocks are 

 normal dolerites, as shown both by their mineralogic composition and 

 textures. 



RELATIONS OF THE GABBRO TO ADJACENT FORMATIONS. 



Within this district the gabbro lies adjacent to the following formations, 

 given in order of age from below up: 



The Ely greenstone (Archean). 



Lower Huronian sediments: Ogishke conglomerate and Knife Lake 

 slates. 



Giants Range, Snowbank, and Cacaquabic granites. 



Upper Huronian sediments: Gruufiint and Rove formations. 



Relations to the Ely greenstone. — The gabbro cuts across the greenstone 

 anticline which occurs to the east of Disappointment Lake, and is also 

 in close proximity to but not in absolute contact with the greenstone 

 of the Twin Peaks anticline on the southwest side of Gobbemichigamma 

 Lake. In both of these cases the greenstone has been metamorphosed by 

 the gabbro, showing conclusively the relative epoch of formation of the 

 two rocks. 



