224 PROCEEDINGS OF THE ACADEMY OF [1888.. 



nine miles from Des Moines, and the second three miles north of 

 Mitchellville, or sixteen miles from Des Moines. 



Drift and carbonaceous clays . . . 110 feet. 



Shale 60 " 



Sandstone 15 " 



Coal IJ " 



Shale ........ 15 " 



Coal 4 " 



A boring near INIitchellville at the eastern border of Polk county 

 shows an almost entire absence of coal. 



Drift 64 feet. 



Blue and black shales with a thin band of 



limestone and one of sandstone . . 17^ " 



Impure coal . . . . . . lo: " 



Gray, black, blue and sandy shales with two 



layers of sandstone .... 141 i^ '' 



Limestone, Avith marly partings . . . 392 " 



Coals No. 2 and especially No. 3, are the most profitably worked 

 and furnish nearly all the coal mined in the county. Immediately 

 overlying, and thus forming the roof of, coal No. 3 is a soft black 

 clayey shale often slaty in places, highly fossiliferous and containing 

 much iron pyrites in the form of crystals and nodules ; many cubes 

 of the former being over an inch along the edges, and the hitter often 

 containino: shells of mollusca. The shell substance of the fossils 

 from these shales, aside from those contained in the pyritiferous nod- 

 ules, is replaced more or less completely by pyrite. In some speci- 

 mens the replacement is complete ; in others only a thin film of 

 pyrite covers the shell, leaving the interior of the shell substance 

 Avith the original calcareous constituents ; between the two extremes 

 all degrees of replacement by pyrite occur. In a few instances — 

 Lophopliyllum, fish-teeth and the remains of crinoids — no replacement 

 has taken place. These fossiliferous shales are, vipon exposure to the 

 weather, easily and speedily disintegrated into a fine black clay, 

 and the iron pyrite contained quickly decomposes; thus render- 

 ing it extremely difiicult to obtain good specimens of fossils, unless- 

 the shales are examined immediately upon being taken from the 

 mines. This fact may account, in part, for the apparent rarity of 

 fossils from the lower coal measures of Central Iowa, as all traces of 

 fossil remains are quickly obliterated after the shales have been 

 disturbed. 



Independent of its biological and geological relations, the fauna of 

 the lower coal measures of Des INIoines is of considerable interest in 

 its bearing upon the geographical distribution during carboniferous 



