1888.] NATURAL SCIENCES OF PHILADELPHIA. 223 



with one or two intercalated bands of impure nodular limestone. 

 These variegated shales have a thickness of forty or more feet, and 

 are easily recognizable at numerous exposures in the bluffs of the 

 vicinity by the thin limestone bands, which within the city limits 

 have yielded twenty or more species of fossils. There are also 

 included in the middle coal measures some local depositions of 

 micaceous sandstone, usually soft, and unfit even for the roughest 

 masonry ; some of it, however, is concretionary and quite durable. 

 Formerly these portions were quarried for local use, but of late no 

 attempt has been made for its utilization. At the southern extrem- 

 ity of Capital Hill this sandstone reaches a thickness of more than 

 twenty-five feet. A short distance north of the city a sandstone 

 having a thickness of twelve feet caps the bluff, and forms a high 

 mural escarpment along the south side of the Des Moines river. 

 Although the Des Moines and Racoon rivers have, in Polk county, 

 corraded their channels through the upper strata, the lower coal 

 measures are fully represented from the underlying St. Louis lime- 

 stone^ — the nearest exposure of which is about thirty miles below 

 Des Moines — to the superimposing variegated shales just mentioned. 

 This formation as represented in this vicinity is composed almost 

 entirely of clays and shales, with a few thin layers of soft sandstone, 

 and at least three workable beds of coal. The relative positions of 

 the latter are shown in the following sections at the Giant Coal 

 Mine where the fossil forms hereafter mentioned were chiefly 

 collected : 



Drift clay and carbonaceous shales . . 56 feet 



Coal No. 1, 



Shales, etc. ...*.. 



Coal No. 2, 



Shales, lower layers fossiliferous. 



Coal No. 3 4* to 6 " • 



To the southwest, from Capital Hill, the distance between coals 

 No. 2 and No. 3 appears to increase, and the latter vein attains a 

 thickness in some places of seven feet. The coal measures of Iowa 

 have a general dip to the south and west. To the northeast from 

 Des Moines, the coal veins appear to thin out and finally are want- 

 ing, as shown in the accompanying sections ; the first at Altoona, 



1 Vide White on the Unconformability of the coal measures upon the older 

 rocks, etc. Geology of Iowa, Vol. I, p. 225 et seq. 



4 



If 



35 



20i " 

 41 " 



