AND COAL-MEASURES OF IOWA. 103 
Near the coal is a bed of dark limestone, almost black when wet, containing 
Productus semireticulatus, Productus cora, a new species of Phillipsia, and an 
Exschara. 
The relative order of superposition of coal, limestone, shale, and grit, is difficult 
to determine, by reason of the disturbance above referred to. The coal at present 
lies near the edge of the water, covered with a few feet of shale, and nearly on a 
level with the base of the grit escarpment, which is in sight about one or two 
hundred yards down stream. 
For two or three miles beyond the coal-bank, the hills do not present the same 
abrupt appearance as they do below; soon, however, the Iowa sweeps around a 
great easterly bend, and again washes the base of hills of sandstones, more regularly 
bedded than the sds aleiaies ranges below. The oxide of iron is here not so 
much disseminated through the substance of the rock, but is rather collected into 
bands, that fill the joints and seams of stratification. 
For several miles after entering the “Big Woods,” the rocks are only seen at 
two localities, not far apart, where a reddish-yellow limestone is exposed, a few feet 
above the water level, while the high ground is composed chiefly of deposits of 
clay and sand mixed with drift, and boulders. These are the only deposits seen 
for twelve or fourteen miles. About latitude 42° 25’, the carboniferous limestone 
again makes its appearance, at first in a succession of low ledges, ten or fifteen feet 
above the bed of the river, sometimes on one side, and sometimes on the other, 
until finally they form “ dalles,” thirty to forty feet in elevation, between the solid 
walls of which the lowa flows, with a rapid current. The limestone which forms 
the base of these “dalles,” is of a light flesh-colour, possesses an imperfect odlitic 
structure, and contains a small Zerebratula, allied to T. laticostata. The upper third 
is composed of magnesian limestones, which have much the appearance of the 
magnesian limestone of the Dubuque District, but cannot belong to the same period, 
since they form a part of the Subcarboniferous Limestone, and contain that form of 
Syringopora which M. Lesueur named Obstringolina, and which was found in that 
formation in Missouri. About the middle of this range of limestone, near a point 
known as “ The Shower-bath,” the rocks attain their greatest elevation, dipping 
from thence southerly and northerly as much as 7°. 
Numerous chalybeate springs issue from their base, depositing an abundant gela- 
tinous hydrated oxide of iron. 
These strata of carboniferous limestone continue, with little interruption, to the 
Falls or Rapids of the Iowa, situated in latitude 42° 32’ 22”. Two miles above 
this, the bluffs rise to the height of one hundred and thirty feet. At their base, they 
are composed of the semi-odlitic layers before alluded to; the coal-measures proper 
resting 
Feet. TInehes, 
1. Soil and drift beds, not exposed, . ; : : 90 to 100 
2. Gray argillaceous schistose sandstone, : : 20 
3. Bluish argillo-siliceous shale, : E . j 12to 14 
4. Chert layer, : : ‘ 5 
5. White siliceous sath. adore escing feebly, . 4to5 
6. White semi-odlitie limestone, : : = 
