38 BULLETIN OF THE UNIVERSITY OF WISCONSIN 



change his opinion as to the origin of the quartz por- 

 phyry. He, however, made no re-examination of the 

 area, and it must be supposed that he was led to change 

 his views on account of his knowledge of the extensive 

 eruptive areas in the Lake Superior region. 



II Field Geology. 



The accompanying map, Plate 1, shows the relations and 

 extent of the different rock formations which occur in the 

 vicinity of the Lower Narrows. Four distinct types of 

 rock are seen to outcrop, representing the quartz kerato- 

 phyre, Upper Huronian quartzite, Potsdam sandstone and 

 conglomerate, and the Lower Magnesian limestone. The 

 cross sections of Fig. 1 cross the range, and in a meas- 

 ure show the structural relations and vertical extent of 

 these formations. For the purpose of convenience the 

 reverse order will betaken in describing these formations. 



LIMESTONE. 



On the south side of the range, and near its summit, in 

 the northwest i of Sec. 25, is a ledge of dolomitic lime- 

 stone overlying the sandstone. It is non-arenaceous, and 

 contains many small cavities lined with calcite. In determin- 

 ing its stratigraphical position, one is met with difficulties 

 owing to the peculiar fluctuations in level of the horizontal 

 formations in the Baraboo region, but from the Dikello- 

 cephalus fauna which it carries, and from its lithological 

 characters, it has been considered ' as being at least not 

 below the Lower Magnesian. ■ 



SANDSTONE AND CONGLOMERATE. 



Overlying both the older formations of quartzite and 

 quartz keratophyre there occur capping and flanking the 



1 Geol. o£ Wis., Vol. II; pp. 594, 595. 



