334 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



questionable Dresbach sandstone and no less certainly established St. 

 Lawrence limestone and shale. The intervening formation therefore 

 seems to occupy the same stratigraphic position as the Franconia. 

 But its lithological characteristics, except that it also contains con- 

 siderable, though more disseminated glauconite, are quite different 

 from those of the Franconia; and whereas good fossil remains of 

 characteristic types are exceedingly abundant in the Franconia they 

 appear to be much fewer and, so far as could be determined from the 

 handful of fragments then procured, of different species. 



In casting about for a means of determining the problem I thought 

 of an old anticline that extends southwestward from the Baraboo 

 range across southern Wisconsin into Illinois. This axis had pre- 

 viously been found to have had an important effect on the distribu- 

 tion of the Ordovician formations and it seemed worth while to see 

 whether it had not been in existence, and functioning as a barrier, 



SOUTH Or BARABOO PRE- CAM BRIAN RANGES 



WEST 

 Mississippi Rmsj 



Green Village 



Madison and 

 Devils Lake 



SHAKOPEE DOLOMITE 



8HAKOPEE OOLOMITE 



Fig. 3. — Section across southern Wisconsin, showing sequence of Upper Cambrian (St. 

 Croixian, and Ozarkian formations, the apparently similar stratigraphic positions of 

 the Franconia and Mazomanie formations, and the absence of both on the summit of 

 the pre-Cambrian anticline. 



already in the Cambrian. Accordingly, a part of the season of 1916 

 was devoted to following the nearly continuous exposures of Cam- 

 brian rocks in the bluffs and valley walls along the Wisconsin Kiver. 



Beginning at Boscobel and going upstream, the Franconia, in 

 typical development, was found to hold its own for a distance of 

 about 20 miles, when it began slowly to lose thickness by overlap. 

 The succeeding 15 miles, which brought us to the town of Lone Rock, 

 sufficed to pinch the formation out entirely. Beyond Lone Rock, for 

 a distance of about 10 miles, in which we passed through the town 

 of Spring Green, the Franconia is absent, the top of the underlying 

 Dresbach sandstone has risen considerably above the river level and 

 is immediately followed by characteristically fossiliferous shales and 

 limestone of St. Lawrence age. (See fig. 3.) 



Just east of Spring Green the closed contact between the Dresbach 

 and St. Lawrence opens again to receive the wedge of magnesian 

 sandstone whose age was the quest of the undertaking. Where first 



