182 



Contributions to Palaeontology. 



both above and below, the epoch represented (so far a3 now 

 known) by the Potsdam sandstone of New York, and 

 which may still be of the same period. 



The evidence from the undisturbed region of the Mis- 

 sissippi valley shows that the period of the Potsdam sand- 

 stone, or a very considerable part of it, may be represented 

 by a small proportion of arenaceous matter. The section 

 of the rocks of Missouri, given by Prof. Swallow, shows 

 four separate magnesian limestones 1 and three distinct 

 sandstones, below beds which he recognizes as the Bird- 

 seye and Black-river limestones. Taking the upper of 

 these magnesian limestones to represent the base of the 

 Birdseye and Black-river limestones, 2 the upper " Saccha- 



present known, bear so exclusively a primordial character as to lead us to 

 suppose that we have reached the lowest beds. The typical forms of 

 Paradoxides of Braintree, Massachusetts, and of Newfoundland, indicate, 

 on palseontological grounds alone, a lower horizon than any we have reached 

 in New York, Canada, or in the Mississippi valley. 



I have already noticed, in the preceding pages, the relations of some of 

 these Trilobites with Paradoxides in certain parts of their structure. The 

 condensation of generic characters observed in true Paradoxides (as well 

 as in typical Conocephalites and Olenus) seem to be diffused among the 

 generic forms of the sandstone of the Mississippi valley ; on which account 

 alone, I would infer that this fauna is of a later epoch than the oldest pri- 

 mordial fauna. I have made similar observations regarding those forms, 

 sometimes termed Paradoxides, in the older slates of Vermont. 



1 The section given by Professor Swallow is as follows : 



BLA CK RIVER AND BIRDSEYE LIMESTONE. 



m f First Magnesian limestone 190 feet. N 



6 

 o 



Saccharoidal sandstone 125 " 



^ Second Magnesian limestone 230 " 



^ Third sandstone 20 " 



Gd 



" I Fourth Magnesian limestone 300 " 



g <j Second sandstone 70 " 



§ Third Magnesian limestone 350 " 



2 Dr. Shumard has remarked (Geological Report of Missouri, Part ii, p. 

 160), that the fossils of this higher Magnesian limestone "are most like 



