THE UPPEK SLATE MEMBER. 299 



what below its average through the t(^)wnship. The surface width at Tylers 

 fork is almost 2i miles (more accurately is about 12,800 feet.) The dip at 

 this locality is from 70'^ to 80° (upon an average it may be taken as 75°), 

 ivnd the surface width then represents an actual thickness of rock of 12,360 

 feet. Through the township R. 1 W., Wisconsin, the average thickness of 

 the belt is not far from 11,000 feet, and nowhere does it fall below 10,000 

 feet. Though R. 1 E., Wisconsin, the belt gradually and t[uite uniformly 

 n;uTOws, and at the east line of this range has a surface width of about 

 1 jl miles, or 7,260 feet. As the dip is here upon an average close to 80°, 

 this width represents a thickness of about 7,150 feet. This width and 

 thickness is maintained with considerable uniformity through R. 2 E., 

 Wisconsin, to the Montreal I'iver. From the Montreal river, the boundary 

 between Michigan and Wisconsin, there is a continuous narrowing of the 

 member through Ranges 47 W. and 46 W., Michigan, until at the Black 

 river the surface width is but 1,650 feet, and as the dip is lu-re Init 65°, 

 this represents a thickness of only 1,495 feet. Here, as before stated, 

 are the last exposures of this formation, and a short distance east the 

 whole member is cut out by the Keweenawan greenstones. 



As the rocks of the Upper slate. Iron-bearing-, and Quartz-slate mem- 

 bers are entirely conformable, there is exposed throughout the whole 

 extent of the rocks of the upper member the full thickness of the lower 

 members of the series. However, a -very considerable portion of the area 

 which should be occupied b}- the slates of the upper formation is taken by 

 the Keweenaw series. It is not certain, even at Tyler's fork, where the 

 maximum thickness of the formation is exposed, that its total thickness is 

 really shown. If at Tylers fork we have tlie full thickness of this mem- 

 ber, the surface which the upper formation rocks cover in the area in 

 which they occur at all is about *one-half of the area which the}' would 

 cover had tliey not been removed by erosion before the l)eginning of 

 Keweenaw time; for at Tylers fork the member gradually and quite uni- 

 formly naiTOws both east and west until it disappears. The surface then 

 covered by this member may be roughly compared to a triangle, the area 

 of which is one-half that of a rectangle of the same base and altitudt- wliich 

 these rocks would extend over were they not cut oil l)\ flic Keweenaw 



