42- THE PENOKEE IRON-BEARING SERIES. 



Wisconsin Geological Survey for the year 1877, by T. C. Chamberliu, Chief Geologist, 

 pp. 17-25. Madi.son, Wisconsin, 1878. 



The principal work of the season of 1877 inckided (1) the extension 

 of a detailed geological section begun in the vicinity of Penokee gap during 

 the previous year, northward 6^ miles to the railroad track near the cross- 

 ing of Silver Creek, Sec. 10, T. 45, R. 3 W. ; (2) the making of a similar 

 section along Bad river, somewhat farther east ; and (3) a detailed map- 

 ping and magnetic survey of the Penokee range from Bad river to 

 Potato river, T. 45 N., R. 2 E., Wisconsin. 



The plan adopted for this work was to cross the iron belt, which, although quite 

 sinuous in its course, preserves still a general east and west direction, curving more 

 and more toward the north as it is followed eastward — from north to south at distances 

 of about half a mile, using the section lines as much as possible. On each of the 

 crossing lines stations were established at every hundred steps, and at every station 

 the aneroid barometer, the variation of the magnetic needle, and the time were care- 

 fully observed, a simultaneous series of barometrical obser^-ations being carried on at 

 Ashland. The lines were begun at points far enough to the south, on the Laurentiau 

 rocks, to be out of the intluence of the iron or magnetic belt of the Iluroniaii, and 

 were extended northward far enough, not only to be out of the influence of this belt 

 in that direction, but also to determine the presence or absence of any other similar 

 belt or belts. Some of the lines, moreover, were extended farther than the rest, so as 

 to pass on to the next scries of rocks, allusion to which has been made above. Other 

 subordinate lines of observation were frequently run across the sections in an east and 

 west direction, and all the lines were controlled by constant reference to section 

 corners and quarter posts. All outcrops were of course examined and located, and 

 specimens were taken for subsequent study, particular attention being given to the 

 magnetic belt traversing the center of the Penokee range. The largest outcrops 

 are found where the several branches of Bad river break through the range from the 

 southward. At each one of these gorges the work was carried into greater detail, in 

 order that the true succession of the various layers might be made out. 



Many interesting facts were developed during this detailed work, one or two of 

 which may be mentioned here. The exact junction of the Huronian and Laurentiau 

 series was found at the gorge of Potato river, where a clift'-side over 100 feet in 

 height and over half a mile in length is traversed near the middle by the highly 

 inclined contact line between the ''siliceous slate," one of the lower members of tlie 

 Huronian, and a greenish chloritic gneiss of the Laurentiau. The siliceous slate 

 inclines at a high angle to the north, whilst the gneiss layers dip to the south and 

 strike in a direction obii(iue to that of the slate layers. It is worthy of note that the 

 two lowermost layers ol' the Huronian, as seen at Penokee gap and lor many miles to 



