CHOUTEAU LIMESTONE 117 



factorily the deep-well records of the region; and why driller's 

 logs are really more accurate than is commonly claimed for them. 



Viewing the Chouteau limestone strictly as a lithologic unit, 

 delimited with unusual sharpness as it happens, several points 

 are to be especially emphasized. The eastern attenuated margin 

 of the formation very nearly coincides with the course of the Mis- 

 sissippi river from the mouth of the Iowa river to that of the 

 Missouri river. Nowhere does the terrane appear actually to 

 touch the banks of the great stream. Chouteau limestone is re- 

 ported to be represented at several points on the river, as at Lou- 

 isiana 14 and Hannibal, in northeast Missouri, and at Burling- 

 ton 13 , Iowa. The thin bed referred to at these places may repre- 

 sent an earthy phase of the Burlington formation, for in this 

 region the latter formation actually rests in marked unconform- 

 ity upon the Hannibal shales. 



In Iowa, north of the original locality, the Chouteau limestone 

 commonly goes under the title of Kinderhook Beds 10 . The for- 

 mation becomes thicker, reaching a measurement of one hundred 

 fifty feet in the central portions of the state. Near the Minnesota 

 boundary, where the Paleozoics are upturned as one limb of the 

 now truncated arch which once formed the Siouan mountains, 

 the thickness is even greater. The formation, after crossing this 

 great Triassie flexure, probably extends northwestwardly far 

 into Canada. 



Between the Missouri river and the Minnesota state-line, a dis- 

 tance of more than three hundred miles, the Chouteau limestone 

 has a thickness of one hundred to one hundred fifty feet. Numer- 

 ous deep-well records in this belt enable the limestone plate to 

 be traced for a distance of seventy-five miles from its outcrop- 

 ping. 



The axis of the broad syncline lying between Chouteau Springs 

 and Hannibal extends southwestward over the present Ozark 

 dome, which of course did not exist in Early Carboniferous times. 

 When the Kinderhook rocks again appear in southwest Missouri 

 the same tripartite character as presented in the north part of 

 the state seems to hold. At Springfield Swallow's original in- 

 terpretation 17 of the sequence appears to be in the main correct. 

 With the elimination of the so-called Devonian beds of the same 



"Am. Jour. Sci., (3), Vol. XLIV, p. 449, 1892. 



15 Bull. Geo], Soc. America, Vol. Ill, p. 285, 1892. 



"Iowa Geol. Surv., Vol. I, p. 56, 1893. 



"Missouri Geol. Surv., 1st and 2d Ann. Repts., p. 103, 1Sj5. 



