The Saint Peter Sandstone. 83 



of the Ordovician (Lower Silurian) as understood by that 

 term in England and in New York, is not unconditionally 

 asserted. In fact, the Shakopee may be proved to be the 

 lowest member of the Ordovician, when more exactly cor- 

 related. 



CORRELATION. 



The period to which the Saint Peter belongs is by far the 

 most easily determined through its relation to the Galena 

 (Trenton) series, which is richly fossiliferous. The Magne- 

 sian series also affords a means of determination because 

 fairly fossiliferous. James Hall and J. D. Whitney (8) in 

 1858 correlated the Saint Peter, in this indirect way, with 

 the Chazy of New York. Hall again in 1862 and in 1863 

 repeats it, and nearly all geologists have followed his corre- 

 lation, hence the name Trenton, which belongs typically to 

 a New York formation, has been equally applied to parts of 

 the Trenton (Galena) series that overlie the Saint Peter. 

 The Trenton series of New York and Trenton (Galena) series 

 oi the upper Mississippi valley are correlated. There is, 

 nevertheless, diversity of opinion over the question, what 

 part of the Galena ( Trenton ) series is equivalent to the 

 Trenton formation proper of the New York Trenton series ; 

 what part equals the Black River formation ; and what the 

 Chazy. 



By many geologists, the Galena (Trenton) series is 

 believed to include only the equivalents of the Black River 

 limestone and Trenton limestone proper, the equivalents of 

 the former being called Trenton and that of the latter 

 Galena limestone. The Saint Peter thus remains to be corre- 

 lated with the Chazy. 



This correlation cannot be said to be undoubtedly 

 established, but represents the extent of our knowledge. It 

 is here inserted because it takes the place necessarily of a 

 direct comparison of the Chazy and Saint Peter faunas, both 

 of which are relatively meagre. 



The two formations, Shakopee and Oneota, beneath the 

 Saint Peter faunally correlate well with the Upper and 

 Lower Calciferous respectively, which underlie the Chazy 

 proper, and they also correspond in their relation to each 

 other and to the overlying formations. 



