Io6 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



i. Rather hard, unevenly-bedded limestone, containing 



Stromatopora and Faros ites 4 feel. 



2. Grayish or dove-colored fine-grained and somewhat 



brittle limestone 16 " 



3. Granular limestone, somewhat magnesian 3 " 



4. Uniformly bedded magnesian limestone 8 " 



Total 31 feet. 



In no other portion of Iowa is the superficial covering of the rocks 

 thinner than over a large portion of the area occupied by the four 

 counties mapped. Not only are the rocks well exposed on most of 

 the streams which traverse the region, but limited exposures are numer- 

 ous adjacent to them, especially in Floyd County. For this reason, 

 the underlying rocky strata can be studied with facility. 



It may not be out of place here to give a short description of the 

 only rock which overlies the shales at any place, so far as observed. 



On Section 6, in Township 94 N., 18 W., Floyd County, there is an 

 exposure of coarse-grained sandstone, forming, for a short distance, the 

 north margin of a rather deep trough-shaped depression in the Devonian 

 strata. This outlier is a very coarse-grained, friable, silicious sand- 

 stone, interstratified with fine, angular gravel (the angles being more 

 or less rounded), while some of the beds exhibit beautiful examples of 

 ripple-marks.* This rock varies in color from grayish-white to very 

 dark yellowish-brown, and in some places is capped by vesicular con- 

 glomerate. The bed, so far as could be made out, attains a thickness 

 of from thirty to seventy feet, the upper portion of which is thin-bed- 

 ded and very much disturbed and broken up, being tilted at almost 

 every conceivable angle, and overlaid at one place by a grayish-white 

 limestone. The lower portion of the stratum is heavy-bedded, and 

 dips at an angle of about twenty-two degrees, in a direction a little 

 west of north. 



The section shown by Figure 2, across this depression or valley from 

 north to south, will show the position which the sandstone occupies in 

 relation to the depression and the Devonian strata, the thin-bedded 

 horizontal rock representing the Rockford shales, and the heavier 

 strata benefeth representing the subjacent limestones. The trough-like 

 depression, which is in part bordered by this outlier, has an easterly 

 trend, and finally unites with the valley of the Shell Rock. Its average 

 width is about three-fourths of a mile, and its depth from forty to 



* This rock is rather peculiar, and nothing- exactly like it has been observed in other portions 

 of the State. I have referred it to the Cretaceous, with some doubt as to its true age, since it is 

 quite unfossiliferous. 



