LAND AND SEA OSCILLATIONS ULRICH. 333 



merely local variations in contemporary seas and life it was almost 

 impossible to work out the true relations of the beds in the largely 

 drift-covered, and hence discontinuous exposures of the Cambrian 

 rocks. A new viewpoint was required; also closer investigation of 

 bedding planes, greater accuracy in noting the vertical and geo- 

 graphic ranges of particular species and fauna! associations and of 

 particular beds. In short, it was necessary to employ more modern 

 criteria, principles, and methods than had been used before. 



When the work of revising the Paleozoic stratigraphy of Wiscon- 

 sin was begun in 1914 the task seemed relatively simple in view of 

 the success that had attended our investigations in the supposedly 

 more difficult fields in the Appalachian region, about the Cincin- 

 nati and Nashville domes, and the Ozark and Adirondack uplifts. 

 Indeed, the results of the first season's work in Wisconsin were so 

 satisfactory to Doctor Walcott that he decided to publish my revised 

 section in his work on the Dikelocephalid trilobites. 5 As therein 

 given the Upper Cambrian series in the Mississippi Valley is divisible 

 into six lithologically and faunally distinct formations, named from 

 below upward : The Mount Simon sandstone, which rests on pre- 

 Cambrian crystallines, followed in turn by the Eau Claire shale, 

 the Dresbach sandstone, the Franconia (glauconite bearing) sand- 

 stone, the St. Lawrence formation of limestone, shale and sandstone, 

 and the Jordan sandstone. Above these came the Lower Ozarkian 

 Mendota limestone and the Madison sandstone, the last of which 

 is overlain by the Oneota dolomite of the "Lower Magnesian" 

 series. Aside from the determination of the lithologic and faunal 

 sequence of the Cambrian in the western half of the State, the most 

 important improvement brought about by the first season's work 

 was the proof that the Mendota limestone and Madison sandstone 

 are really post-Cambrian formations and not, as had been supposed 

 previously, the eastern representatives of, respectively, the St. Law- 

 rence limestone and the Jordan sandstone of Minnesota. In fact, it 

 was then believed and has since been definitely proved that whereas 

 the St. Lawrence extends uninterruptedly from Minnesota and Iowa 

 across the southern half of Wisconsin and under cover of later 

 formations into northern Illinois, the Mendota limestone is entirely 

 absent to the west of a narrow trough running southeastwardly 

 from the southern slope of the pre-Cambrian Baraboo quartzite 

 range. 



In the following field season of 1915 doubt arose as to the eastward 

 extension of the Franconia formation to and beyond Madison. At 

 this place there is a more or less decidedly calcareous sandstone for- 

 mation, approximately 100 feet in thickness, which lies between un- 



B Dikolocephalus and other genera of the Dikelocephalinae. Smithsonian Misc. Coll. 57, 

 1914. 



