366 The American Geologist. June, looi. 
To understand the situation in'respect to coal a few things 
must be kept in mind. First, as ever}- miner knows, the coal 
is interbedded with sedimentary rocks, usually with sandstones 
and shales. Second, sedimentary rocks were laid down, one 
on the other, one after the other, in slow succession ; and so the 
history of rock deposition in Iowa embraces a very long period 
of time. This history is almost complete from a period earlier 
than the introduction of life on the globe to times when lanil 
plants and animals were well developed. Third, coal was 
formed from land plants of certain types, the plants being pre- 
served so as to be transformed into coal only under peculiar 
and favoring conditions. Fourth, coal plants did not come in- 
to existence until long after the beginning of the record pre- 
served in the geological strata of Iowa. The older rocks, 
therefore, can contain no coal, because they were laid down 
long before any coal plants grew. All the rocks indicated on 
the geological map, Plate II, in Volume X, as Algonkian. Cam- 
brian, Ordovician. Silurian, Devonian, and ]\Iississippian, are 
older than any coal. The coal of Iowa occurs chiefly in the 
Des ]\Ioines formation; a little is found in the Missourian. It 
was while these two formations were in process of accumula- 
tion, not before, that coal plants of sufficientl}' luxuriant growth 
to count for anything existed in Iowa ; and though these plants 
Avere abundant, it was only in certain favored and comparative- 
ly limited localities that the preservation of the plants took 
place so as to form coal. The geological formations of Iowa 
lie one on the other somewhat like the shingles on a roof, ex- 
cept that the oldest and first laid formations extend underneath 
the re§t all the way across the state. The older formations ap- 
pear successively from beneath the latter in going from the 
southwest toward the northeast. The Cambrian sandstones 
that are found in the sides of the valleys near Lansing, lie far 
below the surface at Des ]Moines. .\ well bored at Des !\Ioines 
would pass, in the reverse order of their formation, through 
all the older beds, and would finally reach the Cambrian at a 
depth of about i,6oo feet. All these older beds, and all the indi- 
vidual layers of them, are seen in order, one after the other, be- 
tween Des Moines and Lansing; and so a drill hole at Des 
IMoines could reveal nothing of consequence that might not be 
. learned by careful investigation of the natural surface expos- 
