24 Fossils from the Lower Coal Measures — Keyes. 
imum thickness of more than two hundred feet; but notwith- 
standing the fact that at Des Moines the entire formation 
underlies the city, which is situated just at the eastern border of 
the middle Coal Measures this maximum is perhaps nowhere 
in Polk county attained. The base of the middle Coal Measures 
as characterized by St. John,^ and as is well shown in several 
localities in the immediate vicinity of Des Moines, is composed 
of variegated clays and shales, with one or two intercalated bands 
of impure nodular limestone. These variegated shales have a 
thickness, at Des Moines, of thirty or more feet, and are easily 
recognizable at numerous exposures in the bluffs of the vicinity 
by the thin bands of limestone which, within the city limits, 
have yielded twentv or more species of fossils. Although the 
Des Moines and Raccoon rivers have in Polk county corroded 
their channels through the upper strata, the lower Coal Meas- 
ures 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 
section at the Giant coal mine where the fossil forms hereafter 
noticed were chiefly collected: 
Drift clay and carbonaceous shales 56 feet 
Coal (No. 1.) 4 " 
Shales, etc 20 ft. Cinches 
Coal (No. 2.) 4 " 6 " 
Shales, lower layers fossiliferous 35 feet 
Coal (No. 3.) 4 ft.ein. to6 feet 
The Coal Measures of Iowa have a general dip to the south- 
westward. To the northeast from Des Moines, the coal ap- 
pears to thin out and finally is wanting altogether, as shown in 
the accompanying sections; the first at Altoona, nine miles from 
Des Moines; and the second three miles north of Mitchellville, 
or sixteen miles from Des Moines. 
Drift and Carboniferous clays 110 feet 
Shales : 60 " 
Sand stone 15 " 
1 White's Geology of Iowa, \o\. i, p. 272. 
2 F/^<' White on the " Unconformability of tht Coal Measures upon 
the older rocks," etc. Geology of Iowa, vol. i, p. 225, et seq. 
