NATURAL SCIENCES OF PHILADELPHIA. 417 



figures and descriptions, we are left in doubt whether they should be regarded 

 as distinct species, or as varieties of well known European Jurassic forms. 

 Among those so very closely allied to foreign Jurassic species, may be mention- 

 ed an Ammonite, we have described under the name of X cordiformis, which we 

 now regard as probably identical with A. cordatus of Sowerby, a Gryphwa we 

 have only been able to distinguish as a variety, from G. calceola, Quenstedt, a 

 Pecten scarcely, distinguishable from P. lens, Sowerby, a Modiola very closely 

 allied to 31. cancellata of Goldfuss, a Belemnite, agreeing very nearly with B. 

 excentricus, Blainville, &c. &c. 



At the same time that the fossils of this group of strata are generally so close- 

 ly analogous to known Jurassic species of the old world, they are all clearly 

 distinct from any of those found in our Cretaceous rocks above. In short, their 

 Jurassic age is as susceptible of demonstration, both upon stra'igraphical and 

 palffiontolocrical evidence, as that the succeeding rocks above them, or any part 

 of the Green sands of New Jersey, belong to the Cretaceous epoch. The facts 

 likewise, nearly all, as we have mentioned on former occasions, point to the 

 conclusion that they hold a rather low position in the Jurassic system. 



At nearly all the localities already mentioned, where these well marked 

 Jurassic rocks occur, there is at their base a more or less extensive series of 

 brick red strata, composed of fine grained, areno-argillaceous material, with 

 local intercalated beds of gypsum. These red strata seem to be nearly always 

 destitute of organic remains, but from their position we have been inclined to 

 regard them as probably of Triassic age. During Capt. Raynolds' expedition, 

 however, some fossiliferous seams were found near their base, probably 300 

 feet below the horizon of the beds containing so many Jurassic fossils. Amongst 

 these we recognize our IAngula brevirostra, and Monotis curta, Hall, sp., both of 

 which are common in the beds containing the Jurassic fossils at the Black Hills. 

 From this fact we are inclined to think that at least a large part of the red 

 gypsum-bearing strata of this region, should also be included in the Jurassic 

 system. 



These red beds augment greatly in thickness as we go southward ; and as 

 Dr. Newberry and others have shown, similar, if not equivalent strata, are de- 

 veloped on a grand scale in New Mexico. Whether this vast series of red beds 

 in the south-west belongs to the Jurassic or Triassic epoch, or whether they re- 

 present both in part, are questions it would be wandering from our subject to 

 attempt to discuss here. 



CRETACEOUS ROCKS. 



LOWER SERIES. 



In our paper of March 1858, already cited, we mentioned having recognized 

 fresh or brackish-water shells in Lieut. Warren's collections from the south- 

 west base of the Black Hills, obtained apparently from the base of the Cre- 

 taceous series, of that region. These fossils consist of a Unio, a small Planorbis, 

 and fragments of some small univalves like Paludina, all of which were found 

 associated with imperfect specimens of Ostrea. The beds containing these 

 fossils present a somewhat mixed character, being composed, in part, of light gray 

 clays, and purple argillo-calcareous seams. The fresh-water shells were found 

 in the latter hard seams, which sometimes assume nearly the aspect and composi- 

 tion of a true limestone. This formation rests directly upon the Jurassic 

 strata, and seems to pass beneath the older Cretaceous beds.* 



* At the time we published these facts, we were led by lhe discovery here of fresh- 

 water shells, in such a position, to think that some estuary deposits of doubtful age, near 

 the mouth of Judith River, on the Missouri, from which Dr. Leidy had described some 

 Saurian remains resembling Wealden types, might be older than Tertiary. Later ex- 

 aminations, however, have demonstrated, that the Judith beds contain an entirely different 

 group of fossils from those found in the rock under consideration, and that they are really 

 of Tertiary age, and hold a position at the base of the Great Lignite series of the north- 

 west 



1861.] 



28 



