298 THE PENOKEE lEONBEAHING SERIES. 



separating the formations the exposure is strong!}- clayey, and contains a 

 preponderating amount of fragmental material. The exposure does not 

 continue far enough northward for the rock to liecome completely frag- 

 mental. It tluis appears that the passage from the nonfragmental iron- 

 bearing rocks to the fragmental rocks of the upper slates is not complete in 

 a surface distance of 200 feet, which, at a dip of 57'^. which here prevails, 

 con-esponds to a thickness of rock of about 170 feet. This is the only 

 locality in which the actual change from one belt to the other is so well 

 exposed. It is of course possible that in other localities the change is more 

 abrupt, although, as will be seen, the nature of the rocks at the base of the 

 upper slate belts are such as to render it probable that the change is often 

 a transition. 



Geoffraphical distribution. — The westernmost exposure found in the upper 

 slates is near the center of Sec. 18, T. 44 N., R. 3 W., Wisconsin, while the 

 easternmost exposure is at Black river, Michigan, Sec. 12, T. 47 N., R. 46 W. 

 Probably the rocks of the member extend somewhat beyond these points 

 before being cut off by the Keweenaw rocks. At the western extremity of 

 the belt the rocks on the map are carried to near the north and south 

 qiiarter line of Sec. 14, T. 44 N., R. 4 W., AVisconsin. Wliether they ex- 

 tend beyond this point is not known. At the east end of tlie bSlt the 

 Keweenaw rocks upon the map are represented as cutting off the upper- 

 belt rocks about 1^ miles east of Black river, near the east line of Sec. 7, 

 T. 47 N., R. 45 W., Michigan. While it is not certain just where the 

 Upper slate member first emerges from the Keweenawan eruptives at the 

 west, it is certain that the belt rapidly widens until it attains a great 

 breadth and thickness. At a short distance east of Penokee gap the sur- 

 face width of the belt is about 1| miles, or nearly 9,240 feet. The dijj is 

 here about 70°. This surface width therefore represents a thickness of 

 8,630 feet. From this place the width of the member gradually and uni- 

 formly increases, until at the east line of Range 2 W., Wisconsin, the dis- 

 tance across the belt north and south is about 2 J miles, which represents 

 a width ])erpendicular to tlie strike of the rocks of about 2^ miles. Through 

 R. 1 W., Wisconsin, tliis width is about the average. At Tylers fork the 

 width reaches its maximum^ and 2 miles east of Tylers fork falls some- 



