580 A. W. GRABATJ TYPES OF SEDIMENTARY OVERLAP 



more elevated portions of the old land surface, which, being kept free, 

 on subsidence, from sand accumulations, received directly the deposits 

 of the limestones, which thus overlap the basal sand. 



The Bonneterre beds are granular, highly magnesian limestones, often 

 with chlorite in the basal portion. The contact with the underlying 

 formation seems to be a gradational one, indicating continued subsidence, 

 and therefore advance of the sea. The shaly portion contains Lingulepis 

 cf. lamborni, together with some other fossils, which are regarded as fix- 

 ing the age of this bed as probably Middle Cambric. 



An erosion interval is believed by Ulrich to separate this formation 

 from the next overlying Elvins formation, though the evidence is meager. 

 It consists of an irregularity at the top and the presence of one or more 

 beds of limestone pebbles. The Elvins formation is Upper Cambric, 

 according to its fossils. It consists of shales, shaly limestones, and more 

 •or less earthy dolomites. Locally the contact with the overlying Potosi 

 group appears to be disconformable, but in other cases there seems to be 

 a gradation upward into the Potosi. 



The Potosi is on the whole a shallow-water and perhaps in part conti- 

 nental deposit with conglomeratic layers, sun-cracked beds, and local 

 erosions. The lower beds are dolomitic limestones, while sandstones of a 

 more or less lenslike character occur in the middle portion, sometimes 

 amounting to beds of considerable extent and uniformity (Eoubidoux 

 formation). Upward the series is again terminated by a dolomitic lime- 

 stone (Jefferson City limestone), which in turn is succeeded by the 

 Crystal City sandstone, with occasionally an erosion disconformity be- 

 tween the two. The Joachim limestone, however, which overlies the 

 Crystal City sandstone, forms a continuous depositional series with it. 



While the significance of the basal section appears to be marred by the 

 occurrence of planes of erosion disconformity, it seems nevertheless true 

 that the basal sandstone in this section has risen until it probably lies 

 nearer the top than the bottom of the Middle Cambric series. There 

 seem to have been elevations in the Ozark dome at stated intervals, which 

 caused partial retreat of the sea, followed by a readvance. This is the 

 meaning of the numerous intercalated sandstone beds. 



The Upper Mississippi area — Subdivisions. — In eastern Wisconsin the 

 subdivisions of the basal Paleozoic are, according to Chamberlin, as fol- 

 lows :* 



* Geology of Wisconsin, vol. 2, p. 295. 



