GEOLOGY. 327 



of the world, we have yet only seen, from this region, a single fragment, too 

 imperfect to give any clue to its generic relations. The paucity of some, 

 and entire absence of others, of the more common genera of Mollusca, such 

 as Ostrea, Gryphcea, Exoyyra, etc., in these collections is worthy of notice. 

 Future investigations, it is time, may add more species to our present meagre 

 list of these rare forms, yet it is probable we have here something like an 

 expression of the numerical proportions in which many of the lower types of 

 life existed in these ancient seas. 



Of the one hundred and ninety-one species enumerated in this catalogue, 

 forty -four belong to the Tertiary system, and one hundred and forty -seven to 

 the Cretaceous. None of the former are known to occur in the States, or on 

 the other side of the Atlantic, while, of the Cretaceous species, nine appear 

 to be common to the Nebraska formations and those of the States, and four 

 are identical with forms occurring in the Old World. Of these nine species 

 having so great a geographical range, six, or nearly one-third of all that 

 class of Mollusca contained in the list, belong to the Cephalopoda, while nearly 

 all the remaining one hundred and seventy-six species, which appear to be 

 restricted to the north-west, belong to the Lamellibranchiala and Gasteropoda. 

 This, however, is not so surprising when we bear in mind the fact that the 

 habits and organization of these ancient Mollusca must have been such, from 

 what we know of their existing analogues in our present seas, that the for- 

 mer depended on accident, or feeble locomotive organs, for their gradual 

 distribution over the world from their various centres of creation, while the 

 Cephalopoda, owing to their superior locomotive powers, were capable of 

 wandering freely far out over the most profound parts of the ocean. 



It would, perhaps, be premature to attempt, at the present time, the task 

 of tracing out in much detail the parallelism of the various members of the 

 Cretaceous system in Nebraska, with those of New Jersey and other well- 

 known districts in the States, or with those of the south-western territories ; 

 yet the occurrence of several of the more common and characteristic fossils 

 of the upper two Nebraska formations, such as Ammonites placenta, Scaphites 

 Conradi, Bacculites ovatus, Nautilus Dekayi, etc., in the first and second Green 

 Sand beds and intervening ferruginous stratum of New Jersey, as \vell as 

 in the " Rotten Limestone " of Alabama, clearly indicates the synchronism 

 of these deposits, notwithstanding their widely separated geographical posi- 

 tions. 



That these beds, or formations of the same age, are widely distributed 

 over a vast area of country, extending from near the great bend of the Mis- 

 souri, in lat. 44 15", long. 99 20', westward to, and perhaps beyond, the 

 eastern slope of the Rocky Mountains, and far south into Texas and New 

 Mexico, is highly probable, from the occurrence of their characteristic fos- 

 sils at many widely separated localities in this region. 



ON THE EXISTENCE OF POTSDAM SANDSTONE ON THE EASTERN 

 SLOPE OF THE ROCKY MOUNTAINS. 



Messrs. Meek and Hayden, in a paper published in the Proceedings of the 

 Academy of Natural Sciences, Philadelphia, March, 1858, announce the exist- 

 ence of the Potsdam Sandstone on the eastern slope of the Rocky Mountains, 

 a discovery made during Lieut. Warren's expedition to the Black Hills, in 

 the summer of 18-37. These hills seem to have furnished the key to its exist- 



