98 CARBONIFEROUS LIMESTONES OF IOWA. 
Of this formation it is the middle or coal-bearing portion which furnishes also 
iron-stones, hydraulic limestones, selenite, and beds of gypsum 
Through the Des Moines country, between latitude 41° and jntitude 42° 30’, erys- 
tals of selenite are very generally disseminated, in connexion with impure limestone 
and septaria. The heavy beds of gypsum, however, seem to be local in their distribu- 
tion. Our present discoveries of this valuable mineral are confined to an area of a 
few miles on the Upper Des Moines, just below the Second Correction Line, and 
about six miles below the confluence of that river with its Lizard Fork. At 
that locality the supply may be considered as almost inexhaustible. Its average 
thickness, over an area of two miles square, may be put at twenty feet. This 
would give 2,787,840,000 of cubic feet. Or, as the specific gravity of this plaster 
stone is about 2°2 (giving 137-5 pounds as the weight of a cubic foot of gypsum), 
upwards of a hundred and ninety millions of tons. 
No minerals of value have been discovered in the upper division of the carboni- 
ferous limestones of Iowa. They will yield, however, at various localities, good 
freestone for architectural purposes, grindstones, whetstones, and probably also 
furnace hearth-stones. 
SECTION IV. 
THEIR RANGE, EXTENT, AND BEARING. 
Tue Carboniferous Limestone in Iowa, if we commence where it crosses the Des 
Moines, between the mouth of that river and the Missouri line, ranges north as far 
as the confluence of the Iowa and English Rivers; then, for about forty miles, it is 
lost to surface view ; reappearing, on the Iowa River, in Tama County, and ranging 
thence, in a northwesterly direction, towards the head of that stream. There, how- 
ever, it is, to a considerable extent, covered up from view by the drift, showing 
itself, in such cases, only in the cuts of streams. From the source of the Iowa, it 
sweeps off, in a westerly curve, towards the Missouri; its northern boundary, on 
that. stream, being a few miles below the mouth of the Sioux River. Thence it 
bears south, down the valley of the Missouri, to the State line. 
This zone of limestone has an average width of twenty-five miles ; it circumscribes, 
with a short interval, the great coal-field which occupies the whole of Southwestern 
Towa, extending north to latitude 42° 30’; and separates it from the Illinois coal- 
field by a calcareous belt, varying in $d from twenty-five to fifty miles. 
Of this coal-field (in Towa alone, not including its extension south into Missouri), 
the dimensions are as follows. Its average width, from east to west, is less than two 
hundred miles; its greatest length from north to south, about one hundred and 
forty miles; its contents, about 25,000 square miles. It extends, measured in a 
strata of more than 800 feet; and in the Asbey Coal-field only twenty-one, in a depth of 1011 fect. 
In these latter coal-measures, in the hundred feet yielding the greatest number of seams over a foot in 
thickness, there are but six. 
