532 F. T. THWAITES 



THE PALEOZOIC SECTION (fIGS. I AND 2) 



General. — The Paleozoic rocks of southern Wisconsin and 

 northern lUinois comprise sediments of Cambrian, Ozarkian, 

 Canadian, Ordovician, Silurian, and Devonian age. The Cambrian 

 rocks are dominantly sandstones; higher in the column dolomite 

 is more conspicuous. All the formations outcrop in Wisconsin, 

 but in Illinois no Cambrian rocks appear at the surface. The 

 tracing of individual formations from the outcrop to beneath cover 

 has been done mainly by following certain markers or key-beds of 

 well-defined lithologic character, rather than by counting down 

 from the surface. For this purpose it was found desirable to 

 construct sections where one horizon is drawn as a straight line, 

 thus eliminating the confusing effect of structure. The sections 

 herewith presented also show the sea level elevations of the wells. 

 In the discussion of the several formations brief notes are given 

 on their distribution and topographic expression. The data 

 presented herewith are of a purely lithologic nature, the intention 

 being to define the formations in such a manner that they can be 

 distinguished by those not familiar with fossils and in wells where 

 the finding of fossils is extremely uncommon. 



Comparatively few of the Wisconsin well records based on 

 samples have been pubHshed;^ in Illinois the reverse is true.^ 

 The present paper does not aim to present the many well records 

 in detail, but only the general results on which the correlations of 

 the deeply buried strata are based. 



DEVONIAN SYSTEM 



Milwaukee Formation 



Distribution. — The Milwaukee formation of Devonian age is 



known only near Milwaukee, Wisconsin, and in a few small adjacent 



areas. On account of the drift cover it shows no surface expression. 



Character. — The Milwaukee formation is mainly dark gray 



dolomitic shale; there are some layers of gray shaly dolomite 



'Samuel Weidman and A. R. Schultz, "The Underground and Surface Water 

 Supplies of Wisconsin," Wisconsin Geol. and Nat. Hist. Survey Bull. 35, 1915. 



^ J. A. Udden, " Some Deep Borings in Illinois," Illinois Geol. Survey Bull. 24, 1914. 

 C. B. Anderson, "The Artesian Waters of Northeastern Illinois," Illinois Geol. Survey 

 Bull. 34, 1919. 



