184 



THE .MAUQUETTE lUOX-BEARING DISTRICT. 



Tlic jxTidotitc and its altereil varieties t'onn an iinmeuse knob viuder- 

 Iving Potsdam sandstone. Since tlie (U)n<^loineratic' lower layers of the 

 fraginental rock contain pebbles of the peridotite, and since a diabase dike 

 in the latter is overlain bv horizoutal layers of the sandstone, it is concluded 

 that the peridotite is older than the sandstone. Its age with respect to the 

 Algonkian fragmentals is not yet known. It is probabl)' younger tlian 

 the green schists, for it is similar in all essential features to the peridotite 

 of the Opin area, and this intrudes the Kitchi schists. 



Analyses of the serpentine of Presque Isle were made by Whitney' in 

 1859. Thev are of those portions of the rock that were decomposed by 

 fused sodium carbonate, and are evidently not complete. 



Analyses of serpentine from I'reaque Isle. 



THE OPIN AREA. 



The rocks in the vicinity of Opin Lake, in T. 48 N., R. 27 W. (Atlas 

 Sheets XXI and XXIV), that have been classed as peridotites and serpen- 

 tines, embrace also dolomitic serpentine and almost pure dolomites. Phases 

 intermediate between these four types are very numerous, so that there is no 

 difficultv in tracing almost iiure peridotites through serpentinous varieties 

 into pure serpentines and these into dolomites. 



The Opin area includes a number of detached exposures, the larger from 

 1 to 3 miles long, surrounded by green schists and greenstone-conglomer- 

 ates. The combined areas, each composed of from few to many boss-like 

 knobs, trend diagcmally across the various belts of schist. The peri- 

 dotite is never schistose except along shear-planes. Both acid and basic 



'Notice of new localities and interesting varieties of minerals in the Lake Superior region, 

 by J. D. Whitney : Am. .lour. .Sci., Vol. XXVIII, 1859, p. 18. 



