UPPER HURONIAN. 393 



This series of sediments has been divided by Grant," of the Minnesota 

 survey into a "black slate member," with a "graywacke slate member" 

 above it. In our work no attempt has been made to discriminate between 

 these two petrographic facies of the Rove formation. They are not 

 separable by any time interval, but represent merely slight changes in the 

 conditions of deposition. Macroscopically they are very fine-grained black 

 carbonaceous slates, grading up into dark-gray graywackes of medium 

 grain, with occasional bands of material almost sufficiently pure to be called 

 quartzite. In no case were any conglomerates, even fine-grained ones, 

 found associated with these. The slates are unquestionably the predomi- 

 nant kind of rock in the Vermilion district. They are commonly very 

 fissile, although in places these carbonaceous rocks are fairly massive. 



Microscopic characters. — The Rove sediments are composed of angular 

 quartz and feldspar grains in a dark cement. In some cases the character 

 of this cement can be partly seen, and one can then recognize shreds of 

 biotite and chlorite. Between these is a very fine-grained dark material 

 which is presumed to consist of minute dust particles of quartz and feld- 

 spar and ferruginous and carbonaceous material. Many of these rocks 

 are so well crystallized that they may almost be called phyllites. In these 

 crystalline rocks the material between the grains, probably formed from 

 the decomposition of the fine matrix above referred to, consists of flakes 

 of biotite and chlorite, with quartz and ferruginous matter. 



CONTACT METAMORPHISM OF THE ROVE SLATE. 



The Rove slate, as has already been stated, is in contact on its 

 southern border with the Duluth gabbro. At numerous places within 

 the formation there are great intrusive sills which are considered to be 

 offshoots from the Duluth gabbro. The reasons for this view will be given 

 in a later chapter. The gabbro and the sills have had a slight contact 

 effect upon the slates adjacent to them. Actual contacts of the sills with 

 the slates in this district were not seen, but a number of contacts of similar 

 sills on similar slates were seen along the Lake Superior shore in the 

 Thunder Bay district of Canada, and in all such cases the slates merely 

 showed a slight induration. Outside of the district, as, for instance, on Pigeon 



"Geol. and Nat. Hist. Survey of Minnesota, Twenty-second Ann. Kept., 189.3, p. 74; Final Kept., 

 Vol. IV, 1899, p. 470. 



