104 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



At Marble Rock, the limestone beneath the shales is well exposed in 

 the bank of the Shell Rock, with a thickness of ten to fifteen feet or 

 more, and is, for the most part, a rather thick-bedded, hard and fine- 

 grained brittle limestone, varying from nearly white to dark brown in 

 color, with scarcely a trace of fossils. 



The rocks which underlie the shales adjacent to Rudd are thin-bed- 

 ded, fine-grained and brittle limestones, varying in color from nearly 

 white to dark gray, and almost devoid of organic remains. 



About one mile below Charles City, in the eastern part of the county, 

 the beds below the shales are well exposed at the "Marble Quarries" in 

 the east bank of the Cedar River. The banks here rise to a height of 

 about forty or fifty feet above the water in the river. The upper five or 

 six feet is made up of rather thin-bedded, compact (sometimes brittle and 

 crystalline) yellowish-gray limestone. The remaining portion of the 

 outcrop is composed of hard, thicker-bedded, yellow or grayish-brown 

 limestone (and sun-cracked shales), which is literally crowded with 

 Stromatopora, and a few species of the more common Brachiopoda.* 

 At this locality is quarried the rock from which the " Charles City 

 Marble" is manufactured; the rock, as well as the masses of Stromato- 

 pora, taking a fine polish. 



On the west side of the river, one mile above Charles City, there is 

 an exposure of about ten feet of very hard, fine-grained and brittle 

 limestone, with a more or less perfect conchoidal fracture. The layers 

 vary in thickness from five to thirteen inches, and resemble each other 

 so closely that they can hardly be distinguished in hand specimens, 

 especially as all are unfossiliferous. 



At numerous localities along the Cedar, for a distance of sixteen 

 miles above Charles City, the limestone is well exposed, but as it 

 approaches so nearly, in lithological characters and absence of fossils, 

 that of the last section, detailed description is unnecessary.! 



On the Cedar River in Mitchell County, the rocks beneath the shales 

 are well exposed about one and a half miles west of Osage, the ex- 

 posures ranging from forty to more than one hundred feel. The layers 



of Beaver Creek, and, so far as could be observed, reach a thickness of only one or two feet. 

 At Nora Springs, nine miles above Rockford, the same rock is seen to have a thickness of about 

 seven feet; while two and one-half miles south, at the " Old Rowlev Quarry,' 1 in the east bank of 

 the Shell Rock, it attains a thickness of u iward of eighteen feet. At these localities were col- 

 lected S. diy'uncla, S. chemungensis, O. io-coensis, A. Iiystrix, A. reticularis, S. ciemissa, Slrob- 

 ilocystites calvini, etc. 



* As will be seen by referring- to the map, this is one of the few localities where fossil re- 

 mains, in any considerable numbers, are found in the rocks underlying the shales. 



| These rocks differ from those described in the last section only in being more or less crys- 

 talline. 



