GEOLOGICAL INTRODUCTION 21 



some important dolomitic beds in its lower portion, but such beds are 

 much less conspicuous further south. A peculiar topographic feature 

 marking much of the area which is immediately underlain by the St. 

 Louis limestone, is the remarkable abundance of sink-holes; indeed in 

 some portions of the State the boundaries of the formation can almost 

 be drawn by outlining the areas of sink-hole topography. 



The formation has a thickness of from 30 to 40 feet in southeastern 

 Iowa ; it increases in thickness to the south, attaining a thickness of some 

 250 feet in the river bluffs above Alton, and is said to be 325 feet thick 

 in the St. Louis quadrangle. 1 In Monroe and Randolph counties the 

 thickness is not so great as that recorded in St. Louis. In southeastern 

 Iowa the formation is largely a brecciated limestone, and in the bluffs 

 above Alton, a similar brecciated bed about 20 feet in thickness is present 

 near the middle of the formation. In the St. Louis quadrangle brecciated 

 beds are present here and there at different horizons in the formation, 

 but further south this feature has not been observed. 



The fauna of the St. Louis limestone is commonly not so large nor 

 so well preserved as that of the Salem limestone, and in many places 

 determinable fossils cannot be secured through considerable thicknesses 

 of strata. The most varied faunas have usually been obtained from some 

 of the cherty beds. A species of the bryozoan genus Cystodictya is one 

 of the commonest fossils of the formation, and in many places occurs in 

 great numbers upon some of the bedding planes. The corals of the genus 

 Lithostrotion, L. canadense and L. proliferum, are good index fossils, and 

 where they occur they are as a rule present in abundance, locally almost 

 entirely constituting beds a foot or more in thickness, but elsewhere, 

 over considerable areas and through considerable thicknesses of strata, 

 these corals may not be found at all. 



IV. STE. GENEVIEVE LIMESTONE 



Shumard 2 first differentiated the strata immediately above the St. 

 Louis limestone, and gave to them a distinct formation name, the Ste. 

 Genevieve limestone, although he failed to point out that the two forma- 

 tions were separated by a distinct line of unconformity. This uncon- 

 formity has been seen best in the Mississippi River bluffs below Ste. 

 Genevieve, Missouri, where the actual contact of the formation upon the 

 subjacent St. Louis limestone may be observed at several localities, 

 showing the uneven surface of the lower formation, and in places solution 

 channels along joint planes which have been filled with the younger 

 formation. 



iBull. U. S. Geol. Survey, No. 438, p. 23 (1911). 



2 Trans. St. Louis Acad. Sci., vol. 1, p- 406 (1857); Ma Geol. Surv., Kept, for 

 1855-1871, p. 293 (1873). 



