72 UlyRICH: MAJOR CAUSES OF OSCIIvLATlONS 



or at best only very imperfectly understood. Because of cer- 

 tain misapprehensions, now clearly understood, the correla- 

 tions of the several sections by the State geologists of Wisconsin, 

 Minnesota, and Iowa were not only inadequate but quite in 

 error. 



So long as the observed variations in character of deposits 

 and their fossil faunas were supposed to indicate nothing more 

 than 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 geographic ranges of particular species and 

 faunal associations and of particular beds. In short, it was 

 necessary to employ more modern criteria, principles, and meth- 

 ods than had been used before. 



When the work of revising the Paleozoic stratigraphy of Wis- 

 consin was begun in 19 14, the task seemed relatively simple 

 in view of the success that had attended our investigations in 

 the supposedly more difficult fields in tke Appalachian region, 

 about the Cincinnati 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 

 DikelocephaHd trilobites.' As therein given, the Upper Cam- 

 brian series in the Mississippi valley is divisible into six litho- 

 logically and faunally distinct formations, named from below up- 

 wards: the Mt. Simon sandstone, which rests on pre-Cambrian 

 crystallines, followed in turn by the Eau Claire shale, the Dres- 

 bach sandstone, the Franconia (glauconite bearing) sand- 

 stone, the St. Lawrence formation of limestone, shale and sand- 

 stone, and the Jordan sandstone. Above these came the 

 Lower Ozarkian Mendota limestone and the Madison sand- 

 stone, the last of which is overlain by the Oneota dolomite of 

 the "Lower Magnesian" series. Aside from the determination 



5 Dikelocephalus and other genera of the Dikelocephahnae. Smith. Misc. Coll. 

 57: 1914- 



