IOWA ACADEMY OP SCIENCES. 53 



massive, barren, highly dolomitized aspects of both stages 

 that, taken by themselves, cannot be differentiated in the field. 

 Under such circumstances the observer must work out the strati- 

 graphic relations of the particular group of strata under con- 

 sideration before referring it to its place in the geological col- 

 umn. In general the Le Claire limestone is a heavy bedded, 

 highly crystalline dolomite. It contains scarcely any chert, and 

 in the lower part there are very few fossils. There are occa- 

 sionally a few specimens of Pentamerus, of the form described 

 as Pentamerus occidentalis Hall, and the principal coral is a 

 long, slender, tortuous Amplexus which is represented only by 

 casts of the vacant or hollow parts of the original corallum. 

 On account of the complete solution of the original structure, 

 the spaces occupied by the solid parts of the corallum are now 

 mere cavities in the limestone. In the upper part of the Le 

 Claire stage small brachiopods abound. They belong to the 

 genera Homeospira, Trematospira, Nucleospira, Rhynchonella, 

 Rhynchotrepa, Atripa, Spirifer, and. probably others. In most 

 cases the fossils have been dissolved out, leaving numerous 

 cavities. The calcareous brachial apparatus of the spire bear- 

 ing genera is often the only part of the original structure rep- 

 resented. No statement can well give any idea of the numbers 

 of the small shells that crowded the sea bottom near the close 

 of the Le Claire stage, nor of the corresponding number of 

 the minute cavities that are now so characteristic a feature of 

 this portion of the Le Claire limestone. In some loca,lities in 

 Cedar county the small brachiopods of this horizon are repre- 

 sented by very perfect casts that were formed by a secondary 

 filling of the cavities left by solution of the original shell. The 

 external characters are thus fairly well reproduced. 



Compared with the beds of the Delaware stage, the Le Claire 

 limestone as a rule lies in more massive ledges, it is more com- 

 pletely dolomitized, and its fracture surfaces exhibit a more 

 perfect crystalline structure. It contains an entirely different 

 fauna, a fauna in which small rhynchonelloid'and spire-bearing 

 brachiopods are conspicuous. Its fossils are never silicified, 

 and, in marked contrast with some portions of the Delaware, 

 its upper part at least is notably free from chert. The Le 

 Claire limestone is the lime burning rock of Sugar Creek, Cedar 

 Valley, Port Byron, and Le Claire. Wherever it occurs it fur- 

 nishes material for the manufacture of the highest quality of 

 lime. 



