AND COAL-MEASURES OF IOWA. 



103 



Near the coal is a bed of dark limestone, almost black when wet, containing 

 Productus semireticulatus, Productus com, a new species of Phillvpsia, and an 

 Eschar a. 



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 corresponding 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 Iowa flows, with a rapid current. The limestone which forms 

 the base of these " dalles," is of a light flesh-colour, possesses an imperfect oolitic 

 structure, and contains a small TerehratuJa, allied to T. Jaiicostata. 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 Obstringdlina, 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-oolitic layers before alluded to ; the coal-measures proper 

 resting on them in the following order of superposition, from above downwards : — 



1. Soil and drift beds, not exposed, 



2. Gray argillaceous schistose sandstone, 



3. Bluish argillo-siliceous shale, 



4. Chert layer, 



5. White siliceous rock, effervescing feebly, 



6. White semi-oolitic limestone, 



90 to 100 

 20 



12 to 14 



4 to 5 



