130 TWENTY-SEVENTH KEPOET ON THE STATE MUSEUM. 



mitic ; and it seems a most extravagant supposition that the 

 slight lithological differences in the composition of the strata 

 could produce an entire change in the fauna, presuming the 

 deposits to be of the same age. 



"We now come to the consideration of the last paragraph of this 

 remarkable paper, in which we have the following summary : 



"To recapitulate, then, the facts as they are presented in the 

 west ; we find that the dolomites of northern Illinois contain 

 only Niagara fossils, and the siliceous limestones of the south- 

 ern portion of the State, only those considered characteristic 

 of the Lower Helderberg group : while the beds in Tennessee, 

 occupying the same stratigraphical position with the dolomites 

 and the siliceous limestones of Illinois, have Niagara and 

 Lower Helderberg fossils mingled indiscriminately through 

 the strata. Hence we conclude" that the so-called Lower Hel- 

 derberg group has no real existence as a distinct group of Upper 

 Silurian strata, and that the name, being superfluous, should 

 be dropped from the nomenclature of the American rocks." 



The value of this conclusion will be best appreciated from 

 the fact that in southern Illinois and adjacent parts of Missouri 

 the limestones holding the characteristic Niagara fossils lie 

 beneath those containing the characteristic Lower Helderberg 

 fossils ; and that we never " have Niagara and Lower Helder- 

 berg fossils indiscriminately mingled through the strata ; " 

 unless it be in the debris along the outcrop ; and I assert this 

 from my own observation. The same is true of the beds in 

 Tennessee ; and though the collections of fossils made on the 

 outcrops and among the debris do contain fossils of the Niag- 

 ara and Lower Helderberg formations mingled together, this 

 is not true of the rocks in situ. In this opinion I do not rest 

 alone ; and it is only necessary to consult the report of Pro- 

 fessor Safford to show that he finds both the rocks and fossils 

 of the Lower Helderberg formation distinctly separated from, 

 and lying above, those of the Niagara group. 



In some localities Professor Safford asserts that he finds 

 fossils of the two formations mingling along the line of con- 

 tact, which, in the absence of all intervening beds, may very 

 well happen. And this fact, so far from proving the identity 

 or synchronism of the formations, is a very important proof 

 of their distinction in order and in time.* 



* We may inquire, also, whether it may not be inferred that the living organisms 

 of the Lower Helderberg period were spread over a sea bottom covered with the 

 dead organisms of the preceding period and became mingled in this manner. 



