4r)8 THE PENOKEE IROlN-BEARING SERIES. 



most one at which it is probable that rocks of this series will be found. As 

 a matter of fact, the westernmost known ex])osure is in the SE. \ of Sec. 

 24, T. 44 N., R. 6 W., Wisconsin, al)ont 11 miles north of east of this 

 point. In Sec. IG, T. 44 N., R. 5 W., Wisconsin, the eruptive rocks of 

 the Keweenaw series at one point are immediately north of the rocks 

 of the Iron -bearing member, while between them there is little space 

 in this township for the rocks of the Upper slate mend)er. Proceeding east- 

 ward the exposures found in the NE. J of Sec. 23, T. 44 N., R. 5. W., Wis- 

 consin, are the last known in the Penokee series for fully G miles. For 

 this distance the country is low and swampy. The next exposures to the 

 eastward are found in Sec. 24, T. 44 N., R. 4 W., Wisconsin. No attempt 

 is made to map the Penokee series in this interval, as the only indications 

 of its existence are somewhat feeble magnetic attractions,^ which may be 

 due to the magnetite of a basic eruptive. It is not even certain but 

 that the entire succession is here cut off by the overlying series. There 

 are numerous ex[)osures of coarse gabbro in the southern part of Sec. 24, 

 T. 44 N., R. T) W., Wisconsin (see PI. v), a short distance east of the 

 granites in the south i)art of Sec. 23. In Sees. 22 and 23, T. 44 N., R. 

 4 W., are again exposures of gabbros a short distance from the granite 

 to the south and ea^t. We have no evidence which will enable- us 

 to locate these «Teenstones as a oart of the Keweenaw series or as intrusive 

 rocks in the Penokee or underlying series. If they are regarded as 

 Keweenaw rocks, that series must here be conceived not only to take the 

 place of the Penokee succession, but to occupy at least half a mile of the 

 area where one would expect to tind the underlying complex. This struc- 

 ture would imply an erosion not onlv sufficient to remove all of the rocks 

 of the Penokee series, Init to have cut a valley of very considerable depth 

 into the Southern Complex in the interval l)etween Penokee and Keweenaw 

 time. This unusual amount of erosion is not incredible, but the relative 

 locations of the ledges in the south part of Sec. 24 and in the northeast part 

 of Sec. 23 are such as to give an almost incredible steepness to this hypo- 

 thetical gorge. Another possible explanation of the relations is, that the 

 Penokee rocks are absent by erosion and that there has been here a double 



'Geol. ol' Wis., vol. Ill, pp. 278-281. 



