WEBSTER — THE ROCKFORD SHALES OK IOWA. 105 



are of very various lithological characters, varying from yellowish-white 

 and often very heavy-bedded limestone, to dark, grayish-brown crystal- 

 line dolomite, with but very few and poorly preserved fossils. 



In the north-eastern portion of Worth County, on Deer Creek, near 

 Carpenter, the limestone exposed is rather unevenly bedded and slightly 

 friable, varying in color from yellowish-white to dark grayish-brown, and, 

 so far as observed, devoid of fossil remains. 



In lithological character, the rocks outcropping along Rock Creek, 

 in the south-western portion of Mitchell County, are very much like 

 those ^observed at the locality one mile above Charles City, in Floyd 

 County. The beds, however, are much thicker, and contain an abun- 

 dance of fossils, represented by the genera Stromatopora, Cyatho- 

 phyllum, Favosites, and a few of the more common Brachiopoda.* 



On the Shell Rock, at Nora Springs, in Floyd County, there is an 

 exposure of over forty feet of limestone, the beds of which vary some- 

 what in lithological character, but are, for the most part, fine-grained, 

 compact, and more or less brittle limestone (some portions slightly 

 magnesian), crowded with fossils, among which the genera -Favosites, 

 Gyroceras, Paracyclas, Pachyphyllum, Stromatopora, Alveolites, 

 Fenestella. Diphyphyllum, Atrypa, and crinoid remains (representing 

 two or three genera) were observed. t 



At Rock Falls, in the north-eastern part of Cerro Gordo County, 

 four miles above Nora Springs, the beds below the shales are well ex- 

 posed. They consist of thin-bedded, hard, and rather crystalline mag- 

 nesian limestone, varying in color from light gray to nearly black. 

 The most careful search failed to reveal the presence of fossils. The 

 lithological character of the rocks at this place is peculiar, and nothing 

 exactly like it has been observed in any other part of the State. They 

 are all highly magnesian, but not pure dolomites, as they contain 

 rather more lime than belongs to the composition of the double car- 

 bonate of lime and magnesia.;); 



The following section will give an idea of the general lithological 

 character of the rocks underlying the shales in the region of Mason 

 City, in Cerro Gordo County. It was measured on Lime Creek, three- 

 fourths of a mile above Mason City : # 



* By referring- to the map, this will be seen to be the second locality where organic remains 

 are common in the rocks below the shales. 



f Most of the species found here are restricted, in their vertical range, to the lower portion 

 of the exposure; while about fifty per cent of the forms do not, so far as is now known, occur in 

 the shales above. 



\ Hall's Geological Survey of Iowa, Vol. I., Part I., page 310. 



[Proc. D. A. N. 8., Vol. V.J 14 [October 5, 1888.] 



