114 



The Ottawa Naturalist 



[Vol. XXXII. 



5. Yellow unfossiliferous shaly lime- 

 stone 1 



4. Hard, brittle, unfossiliferous grayish 

 limestone 1 



3. Limestone similar to the above in ap- 

 pearance, but very shaly 



2. Thickly bedded gray buff limestone- 1 



I . Heavy yellowish buff to brownish 

 limestone very coarse and apparently 

 unstratified. Porous with large 

 amounts of calcite, and often much 

 broken in portions, unfossiliferous. 

 To bed of creek 3 



same as those shown at the Bloody Run section and 



4 the relative position is the same. In Charles City 



the mud-crack horizon may be found in places below 



6 great beds of the stromatoporoids and beneath the 



water level of the Cedar river. At Cedar Rapids, 



10 in Linn county, the writer observed what was taken 



2 to be a continuation of this horizon though he can 



make only a provisional statement that it occurs 

 there. Mr. Webster considers that this horizon ex- 

 tends even to the south of Cedar Rapids. 



The most satisfactory localities for the study of 

 this horizon are the two Bloody Run quarries. The 



1 bed in these quarries has been exposed for a long 



Fig. 1, A and B Mud-crack; the smaller specimen (A) shows the first type: the larger one (B) 

 shows that of the second. Specimens in collection of Carroll Lane Fenton. 



Bed number 12 of this section has the widest 

 extension of any mud-crack horizon of the Iowa 

 Devonian known to the writer. It is found at var- 

 ious points east of Charles City, at Devonia, in the 

 north part of Floyd county, at and near Osage and 

 Mitchell in Mitchell county, and in other localities in 

 the northern portion of Floyd as well as in portions 

 of Cerro Gordo and Worth counties (C. L. Web- 

 ster). At Waterloo, in Blackhawk county, it is a 

 continuous horizon, and south of the State Teachers' 

 College at Cedar Falls it is also well developed and 

 has a good exposure on the bank of a small creek. 

 The characters at these localities are practically the 



term of years and the underlying rocks have so 

 broken away that large specimens can be secured 

 with comparative ease. The horizon at this point 

 consists of two quite distinct divisions. The lower 

 of these is of extremely thin bedded shale. The 

 polygons are of large size from two to six inches 

 at greatest diameter and the interspaces are often 

 one-fourth to one-half an inch wide (fig. 1,A). 

 The edges of the polygons are often very decidedly 

 downwarped and in the specimen figured this char- 

 acteristic is well developed. 



The second type (fig. 1, B) is one of smaller 

 polygons of more regular form with the greatest 



