INTRODUCTION. XI 



fectly preserved fossils were known from these beds below the 

 sandstone mentioned above, they were regarded as most proba- 

 bly belonging mainly, if not entirely, to the Upper Silurian 

 epoch. During the progress of the detailed county surveys of 

 that region, however, Mr. Engelmann found a few additional 

 fossils, at different horizons above the middle of this doubtful 

 series, which were submitted for examination, and found to 

 indicate that at least a considerable portion of these beds are 

 more nearly allied to the Oriskany sandstone than to the Upper 

 Silurian. The fact, however, that the few specimens, with one 

 exception, then obtained in a condition to be identified, were 

 found to belong to species that are known to be, in New York or 

 Canada, common both to the Oriskany formation and the Lower 

 Helderberg group, left it still a matter of doubt, without farther 

 examinations and the additional collections of fossils, at what 

 precise horizon in this series the line should be drawn between 

 the Devonian and the Upper Silurian. Consequently the whole 

 group between the two horizons mentioned above was, in the 

 first volume, designated as the "Clear-creek limestone" with 

 the statement that the line between the Upper Silurian and 

 Devonian should probably be drawn somewhere through this 

 group. 



Wishing, if possible, to clear up this doubtful point, we 

 availed ourselves of a short, unavoidable delay in the progress 

 of the printing of this volume on Palceontology, to spend a few 

 days in examining these rocks in Union county, and some of 

 the adjoining portions of Missouri. As the lower portion of 

 this doubtful series seems to be nearly destitute of fossils in 

 Illinois, we directed our attention first to outcrops of these beds 

 on the opposite side of the Mississippi, in Perry county, Mis- 

 souri, Avhere they were already known to be fossiliferous. At a 

 point a little below Bailey's Landing, in Perry county, we col- 

 lected a number of fossils, clearly establishing the fact that the 

 whole thickness, of over two hundred feet, exposed there, be- 



