16 MISSISSIPPI AN BRACHIOPODA 



two basins are fundamentally different in composition and origin. With 

 the progress of Kinderhook time this land barrier was gradually sub- 

 merged, causing the borders of the seas upon the two sides to approach 

 each other, until in late Kinderhook time the northern and southern seas 

 joined, and the fauna from the southern sea spread into the northern 

 basin, supplanting the previously existing fauna of that region. 



II. OSAGE GROUP 



The formations constituting the Osage group are very different from 

 those of the Kinderhook in that they are fairly uniform in both lithological 

 and faunal character, and can be traced through the entire Mississippi 

 Basin. The formations commonly included in the Osage are the "Boirling- 

 ton limestone, the Keokuk limestone, and the Warsaw formation. Ulrich, 1 

 however, includes the last of these formations in the superjacent Meramec 

 group, and considers the Fern Glen formation, here placed in the Kinder- 

 hook, as a basal member of the Osage. 



Burlington limestone. The Burlington limestone takes its name from 

 Burlington, Iowa, and the formation has its most typical development in 

 the southeastern counties of Iowa and the adjacent portions of Illinois. 

 Further south the formation is continuously exposed in the Mississippi 

 River bluffs from Quincy to northern Calhoun County, and also in south- 

 ern Jersey County. It has a considerable area of outcrop in the river 

 bluffs and elsewhere in Monroe County, and still further south in Jackson 

 and Union counties. In its typical development in southeastern Iowa, a 

 total thickness of about 150 feet has been recorded for the formation, 2 

 but its thickness commonly falls short of this and in some localities is not 

 over 100 feet. The formation is constituted mainly of limestone and chert 

 in varying proportions. The limestone layers are conspicuously crinoidal, 

 generally consisting of a mass of the separated column joints and plates, 

 and are in many places remarkably pure, locally containing 98 per cent 

 of calcium carbonate. In Iowa the uppermost 30 feet comprise the so- 

 called Montrose chert bed, in which the limestone and chert occur in 

 alternate horizontal bands a few inches in thickness, the chert constituting 

 50 per cent or more of the total mass. Elsewhere in the formation in this 

 region the chert is less conspicuously developed, being present in oc- 

 casional horizontal bands or in horizontal series of concretionary nodules. 

 Further south the chert is not so conspicuously concentrated in the upper 

 portion of the formation, but in many places occurs abundantly through- 

 out the entire thickness, or elsewhere more concentrated near the base. 



1 U. S. Geol. Survey, Prof. Paper, 36, p. 24 (1905) ; Bull. Geol. Soc. America, vol. 

 22, plate 29. (1911.) 



2 Iowa Geol. Survey, vol. Ill, plate 28, opposite p. 330. (1895.) 



