IOWA, AND WAPSINONOX RIVERS. 



85 



A quarter of a mile lower down, near the middle of Section 34, of the same 

 township and range, below Washington Ferry, there is a fine quarry of heavy beds 

 of subcrystalline magnesian limestone. This rock, which is of the Upper Silurian 

 period, dips southwesterly under the thin-bedded limestones above the Ferry. 

 These latter appear, from their chemical composition, to belong to the Devonian 

 System, although no evidence was derived from organic remains, which are very 

 scarce at both localities. Some well-known Devonian forms are, however, in the 

 debris of the river near by. 



In Hickory Grove, on the southeast corner of Section 34, Township 80 north, 

 Range 4 west, of the 5th Principal Meridian, both magnesian limestone and white 

 limestone lie within two yards of each other ; the latter containing Spirifer euru- 

 teines, Gorgorda retiformis (?), and a Stromatopora of the same species as that found 

 in the Winnebago Reserve. 



No boulders were found near Cedar River, in townships 79, 80, and 81. A few 

 only were noticed in the east part of Johnson County, eight miles from Iowa City, 

 near the Dubuque Road. 



The inferences to be deduced from the foregoing observations, made in Iowa on 

 both sides of Cedar River, in Muscatine, Johnson, Cedar, and Linn Counties, are as 

 follows : 



All the rocks, as well those referable to the Upper Silurian, as to the Devonian 

 and Carboniferous periods, have been subjected to disturbances subsequent to the 

 carboniferous era. These disturbances have been chiefly dislocations, through 

 which the strata have been displaced more by abrupt vertical depressions and 

 elevations, than by prolonged, arched, and waved movements. 



The subcarboniferous limestone, which forms a zone around the coal-measures, 

 and occupies the valley of the Mississippi, between latitude 40° and 41°, is lost .to 

 view, for forty miles beyond latitude 41° 25'; reappearing, however, in Tama 

 County. Even those local beds of limestone mentioned in my Report of 1839, 

 containing reticulated lamelliferous corals, and GyatTwpora Ioivensis, which it was 

 thought might be of that age, prove, on a more minute investigation, to be of a 

 type indicative of the period of the Hamilton Group of New York. 



The calcareous beds, which constitute a conspicuous feature of the Lower Coal- 

 measures in the Des Moines Valley, are not traceable here ; the base of the Carbo- 

 niferous System of Muscatine County being arenaceous and argillaceous grits, 

 characterized by different species of Lepidodendron, and very large globular con- 

 cretions.* 



The Devonian rocks consist chiefly of close-textured white or gray limestones, 

 sometimes brecciated, or of argillaceous limestones, both varieties containing a 

 much smaller percentage of magnesia than the adjacent dolomitic rocks of Upper 

 Silurian date. The former are of no great thickness, probably not exceeding 

 seventy feet. 



Bocks of the Ioiva River. — On Section 10, Township 79 north, Range G west, of 

 the 5th Principal Meridian, on the east side of the Iowa River, on the town plot 



* See sketch at Muscatine Quarry. 



