SILUEIAN. 243 



The Le Claire stage of the Niagara follows the Delaware. The exact line of separation 

 between the two stages has not been and probably can not be definitely drawn. There are 

 massive, barren, highly dolomitized aspects of both stages that, taken by themselves, can not 

 be differentiated in the field. Under such circumstances the observer must work out the 

 stratigraphic relations of the particular group of strata under consideration before referring it 

 to its place in the geological column. In general the Le Claire limestone is a heavy-bedded, 

 highly crystaUine dolomite. It contains scarcely any chert, and in the lower part there are 

 ■ very few fossils. There are occasionally a few specimens of Pentamerus, of the form described 

 as Pentamerus ocddentalis 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 coraUum. On 

 account of the complete solution of the original structure, the spaces occupied by the solid parts 

 of the co^allum 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, 

 RhynchoneUa, 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-bearing genera is often the only part of the original structure represented. 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 hmestone. In some localities jn Cedar 

 County the small brachiopods of this horizon are represented by very perfect casts that were 

 formed by a secondary filluig 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 completely dolomitized, and its fracture surfaces exhibit a more perfect 

 crystalline structure. It contains an entirely different fauna, a fauna in which small rhynchonel- 

 loid 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 in some respects unique among the geological formations of 

 Iowa. In the first place it varies locally in thickness, so much so that its upper surface is exceed- 

 ingly undulating, the curves in some places being very sharp and abrupt. In the second place 

 it differs from every other limestone of Iowa in frequently exhibiting the peculiarity of being 

 obliquely bedded on a large scale, the oblique bedding often affecting a thickness of 15 or 20 

 feet. The phenomena suggest that during the deposition of the Le Claire limestone the sea 

 covered only the. southwestern part of the Niagara area, that at times the waters were com- 

 paratively shallow, and that strong currents, acting sometimes in one direction and sometimes 

 in another, swept the calcareous mud back and forth, piling it up in the eddies in lenticular 

 heaps or building it up in obliquely bedded masses over areas of considerable extent. The 

 oblique beds observe no regularity with respect to either the angle or direction of dip. Within 

 comparatively short distances they may be found inclining to all points of the compass. Again 

 the waters at times were quiet, and ordinary processes of deposition went on over the irregular 

 sea bottom, the beds produced under such circumstances conforming to the undulating surface 

 on which they were laid down. » 



In Illinois and Wisconsin the Silurian is known simply as the Niagara dolomite 

 or limestone, the formation varying in lithology in different areas. The formation 

 in Illinois is thus described by Bain: ^^'^ 



Above the soft shale of the Maquoketa is a fine-grained dolomite, carrying more or less 

 chert. This forms the uppermost member of the stratigraphic section in this area. The beds 

 in the lead and zinc district form only the lower portion of the great thickness of strata which 

 have long been known collectively as the "Niagara." There is no established local name for 

 these particular beds, and since they were doubtless formed at some time within the "Niagara" 



