216 GENERAL GEOLOGY. 



northerly ones at which the formation is exposed, and they 

 are widely isolated from the principal portion of the area it 

 occupies in Iowa; between which area, however, and those 

 northerly points it appears by a small exposure in the valley 

 of a tributary of Skunk river, near Ames, in Story county. 



At Sternberg's mill, on Boone river, Fl »- 9 - 



six miles below Webster city, the 

 following section, illustrated by fig. 

 9, was measured, commencing at the 

 water-level below the mill-dam : 



No. 3. Compact, gray, fragmentary, and concretionary limestone. 4^ feet. 



No. 2. Soft, clear-grit grayish sandstone 4 feet. 



No. 1. Shaly calcareous sandstone 10 feet. 



No. 3 contains fossils characteristic of the St. Louis limestone, and although 

 none of any kind were found in the other strata they are also referred to the 

 same formation without hesitation. A full section of the rocks at Fort 

 Dodge, including the St. Louis limestone there, the coal-measures, and the 

 gypsum will be found in connection with a description of the geology of 

 Webster county. 



The eastern part of the Lower coal-measure formation is 

 so thin that the larger streams which traverse the region it 

 occupies, have cut their valleys down entirely through it and 

 exposed the St. Louis limestone floor upon which it rests. It 

 is found exposed in this manner at frequent intervals in the 

 valley of the Des Moines river from a point near the centre of 

 Marion county to the mouth of the river. This is also the 

 case in a part of the valley of Skunk river and of some of its 

 larger tributaries. 



The St. Louis limestone formation, as it exists in Iowa, 

 consists of three tolerably distinct sub-divisions, princi- 

 pally dependent on lithological characters. These are 

 magnesian, arenaceous, and calcareous. The first and 

 lowest consists of a series of yellowish gray, more or less 

 magnesian and usually massive layers. The second is a 

 yellowish or light gray friable sandstone, and the third 

 or upper division is principally composed of light gray 



