400 Coal Measures of Central Iowa — Keyes. 
south-westward. The lower coal measures which over a greater 
part of this region are covered by drift, pass beneath, in the 
south-western portion of Polk coimty,the middle coal measures, 
the most eastern extension of which is approximately at Des 
Moines. In Iowa the lower coal measures have probably a 
maximum stratigraphical thickness of over two hundred and 
fifty feet, but in the county, though this formation is completely 
represented, from the underlying St. Louis limestone to the 
superimposing variegated shales of the middle coal measures, 
this maximum is perhaps nowhere attained. The productive 
coal measures of the region under consideration are, as shown 
clearly in the accompanying sections, composed almost entirely 
of clays and shales, with a few unimportant beds of friable sand- 
stone and, in the proximity of Des Moines,at least three persist- 
ent coal seams, besides other irregular and thinner carbona- 
ceous layers. The section at the Giant coal mine is a typical 
one for the immediate vicinity of Des Moines, though at dif-^ 
ferent places the thickness of the three workable coal beds, and 
their distance from each other varies somewhat. Coal No. 1 
(Section V) although in many places four feet in thickness, is 
mined but little, chiefly on account of its somewhat inferior 
quality, the prevalency of clay seams, and the existence only a 
short distance beneath of two better and more profitably worked 
beds. No. 2 and particularly No. 3 are the most extensively 
mined coals, and furnish nearly all of the coal obtained in the 
county, which in 1884 amounted to 620,000 tons. The thickness 
of the former is four feet; that of the latter forms four 
to seven feet. The roof of coal No. 2 is usually much 
better than that of either Nos. 1 or 3, being made up of 
two or three feet of shales, overlaid by fifteen to eighteen feet 
of sandstone. No. 3. is far more valuable economically than 
either of the other two important seams; and occupies a much 
greater area. It is overlaid by a soft bituminous clayey shale, 
often slaty in places, and is highly fossiliferous; * it also contains 
much iron pyrites in the form of crystals and nodules; and the 
fossils contained have the original calcareous material replaced 
mor» or less completely by pyrite. Upon exposure to the at- 
mosphere the pyrite rapidly decomposes and the shales quickly 
* A list of the fossils from these shales will be found in this journal, 
vol . ii, p. 25. 
