308 The American Naturalist. [April, 
and uniform texture, and facility of working, both by hand and 
machinery, make it extremely valuable for architectural purposes, 
and its assured strength and durability make it especially desir- 
able for all permanent engineering works.” 3 
Another notable feature of this limestone, especially in Iowa, 
is its irregularity as to thickness: frequently varying from ten to 
fifty feet within very short distances. At Keokuk the thickness 
is from ten to twenty feet ; following up the Des Moines river, the 
course of which is nearly parallel with its original outcrop, its 
thickness increases until we reach Farmington, where it measures 
seventy-five feet. Between this place and Bentonsport, thirteen 
miles beyond, it decreases to four or six feet. This irregularity: 
in thickness is accompanied by trough or basin-like depressions 
in the surface of the limestone, in which the coal measures were 
afterward deposited. A miniature basin of this kind occurs at 
Hillsbérough, while at Farmington the coal occurs in a more ex- 
tensive depression. At Hillsborough the basin is “ oval in form, 
and does not exceed fifty paces in diameter in either direction. 
The coal dips rapidly from the edge to the centre, where it is 
about fifteen feet below the surface of the limestone, outcropping 
around the rim of the basin.” Fig. 1, Plate X., shows a cross- 
section of this basin : | 
We have observed the same irregularity in the surface of this 
limestone at Keokuk, the thickness at one place being diminished 
by half in a distance of one hundred feet. The accompanying 
section across Point Keokuk from northeast to southwest (Fig. 2, 
Plate X.,) shows the observed position of these beds. 
Toward the northeast the junction of the sandstone with the 
limestone may be observed, showing very conclusively the uneven 
surface of the limestone, and, a few inches above, a black coaly 
layer, here amounting to a mere parting, but which rapidly 
thickens to a layer ten or twelve inches in thickness, accompanied 
by a still greater thickness of slate. This basin is apparently a 
very small one. Within the limits of the city these rocks have 
been mostly removed by erosion. A similar basin occurs toward 
5 Indiana Report, 1881., p. 29, et seq. 
6 Hall’s Iowa Report. Vol. I., Part 1., p- 223. 
