56 GEOLOGICAL FORMATIONS 



Hannibal Shales. — The Hannibal Shales ( Vermicular 

 shales of Swallow ) have a maximum thickness of more than 

 70 feet at the typical locality in Marion county ( plate vi). 

 They are fine bluish or greenish argillaceous beds, often with 

 considerable amounts of calcareous and magnssian carbonates, 

 forming in places impure earthy bands of magnesian limestone. 

 The upper portion usually contains much fine arenaceous 

 material, passing locally into sandy shales, shaly sandstones, 

 and to the northward especially substantial sandrocks suitable 

 for ordinary masonary. The indurated sandstones are largely 

 absent in the southwestern part of the State. Downward the 

 shaly sand-beds lose their arenaceous character and pass 

 rapidly into the greenish clay shales, which appear remarkably 

 uniform over broad areas. At Burlington, Iowa, recent exca- 

 vations show a thickness of more than 70 feet, while borings 

 indicate a vertical measurement of double that figure. 



These shales attain their best development in Marion, 

 Ralls and Pike counties, and in the contiguous districts of 

 Illinois. From this place they pass southeasterly and south- 

 westerly in a broad curved belt around the Ozark uplift. 



In southeastern Missouri the exact equivalents of this 

 formation are not definitely understood. In Ste. Genevieve 

 county, certain variegated clay-shales below the Burlington 

 Limestone have been referred to the Vermicular of Swallow. 

 Worthen has also regarded as Kinderhook a series of argilla- 

 ceous and silicious variegated shales found immediately above 

 the " black shale" of the Devonian, in the adjacent parts of 

 Illinois. 



Westward from the typical locality, the Hannibal shales 

 become more calcareous and much thinner, in some places 

 apparently unrepresented, while the Ohouteau rests directly 

 upon the Ozark series of magnesian limestones. This thin- 

 ning of the formation in question seems to be due in a large 

 part to a low anticline tending northwest and southeast 

 through Pettis, Morgan and Camden counties. The origin of 

 the fold probably dates back to a period soon after the deposi- 

 tion of the last of the Ozark series. South of the anticlinal 



