416 PROCEEDINGS OF THE ACADEMY OP 



then identified by us were collected by one of the writers, (Dr. H.,) who acfed 

 as geologist of Lieut. Warren's expedition. They consist of languid prima, L. 

 antiqua, and an Obolella, with fragments of a Trilobitb of Primordial type, 

 similar to some of those occurring in rocks of that age in Wisconsin. 



In the following pages of this paper we give descriptions of apparently the 

 same Trilobite mentioned above, and of a small Pteropod? of the genus Pugiunculus, 

 from the Primordial or Potsdam Sandstone at a locality near the head of Powder 

 River, on Big Horn Mountains, a part of the Rocky Mountain range, near 43 30 r N. 

 lat. long. 108 W. These fossils were collected by one of the writers, (Dr. Hayden, 

 who acted as geologist of Capt. Raynolds' expedition,) from a brownish some- 

 what laminated sandstone, also containing a Lingula apparently identical with L. 

 antiqua, but smaller than the average size of that shell. At this and other 

 localities along the Rocky Mountains, west of the Black Hills, as well as at the 

 latter, this rock was seen resting either directly upon granitic masses, or 

 ancient upheaved metamorphic slates. At the Black Hills, it is usually only 

 from 50 to 80 feet in thickness, but in the Big Horn Mountains, it sometimes 

 attains a thickness of two hundred feet. 



Up to this time, we have no positive evidence of the existence of any of the 

 usually succeeding Silurian and Devonian rocks, throughout all this region, north 

 of the South Pass, lat. 4231'N. long.109 W. From the latter locality, we have 

 identified specimens of Halgsites catenulata and a few other fossils probably of 

 upper Silurian age.* North of this, however, so far as we know, the Primordial 

 Sandstones, are directly succeeded by heavy deposits of Carboniferous age, 

 of arenaceous and more or less pure limestones. Surmounting the latter there 

 were also seen occasional local beds of raagnesian limestones of the same age, 

 and containing some of the same fossils as those referred by us and others in 

 eastern Kansas, to the Permian epoch. As it is our purpose, however, to con- 

 fine our remarks more particularly to the strata from which the fossils described 

 in this paper were obtained, we pass on to the 



JURASSIC ROCKS. 



In a paper already referred to, (published by us in the March number of the 

 Proceedings for 1858), we announced that we had identified Jurassic types of 

 fossils among the collections brought in from the Black Hills, by Lieut. War- 

 ren's expedition. So far as we know, these were the first true Jurassic fossils 

 ever identified from the region of the Rocky Mountains. In April, 1860, one of 

 the writers, (F. B. M.,) and Mr. Henry Engelmann, recognized some of the same 

 species along with a few new forms, in the collections brought by Capt. 

 Simpson's expedition, from equivalent beds at Red Buttes on the north Platte, 

 and from near Uintah and Weber River, in Utah.f 



The specimens of this age collected during Capt. Raynolds' expeditions are 

 in part from near the head of Wind River Valley, in the Rocky Mountains, lat. 

 43, 30' N., long. 110 W., and from Big Horn Mountains, lat. 43 30' N., 

 long. 108 W. At both of these localities, at the Black Hills, and at the Red 

 Buttes on the north Platte, as well as at the other localities already mentioned 

 in Utah, the rocks containing these Jurassic fossils consist of a series of grayish, 

 ash-colored, and red argillo-calcareous, more or less gritty strata, with beds of 

 soft dark brown, and reddish sandstones. These beds preserve a remarkable 

 uniformity of characters, taken as a group, wherever they have been seen, and 

 need never be confounded with the Cretaceous or Tertiary rocks so widely dis- 

 tributed over the north-western Territories, even where no fossils are to be 

 found. They are usually only seen as we approach the mountains, near which 

 they rise from beneath the Cretaceous strata. 



The organic remains found in this series present, both individually and as a 

 group, very close affinities to those of the Jurassic epoch in the old world ; so 

 close indeed, that in some instances, after the m ost careful comparisons with 



Transac. American Philosophical Society, March 4, 1859, page 137. 

 tSee Proceedings of the Academy, April, 1860, p. 129. 



[Dec. 



