1 882.] Central Region of the United States. 1 9 1 



quantity of remains of Ungnlata, Carnivora, Rodentia, etc. The 

 lithology is as follows : Bed A is a white calcareous soft clay 

 rock, breaking into angular fragments. Bed B has a similar 

 mineral character, with frequently a red color of different obscure 

 shades. Bed C is a sandstone of varying persistence. Bed D 

 is a white argillaceous rock like that of bed A. Fossils are less 

 numerous than in bed B, and included no Symborodons nor 

 other Menodbntidcs. 



The eastern area of this formation is the true White River 

 epoch of Hayden ; the western deposits form the Truckee epoch 

 of King. I named this formation the Oregon, but Mr. King's 

 name is the older and must be retained. 1 



According to Professor Condon, the Truckee formation of 

 Oregon, on the John Day river, rests unconformably on the lami- 

 nated beds, containing Toxodinm and fish remains, which, as I 

 have suggested on a previous page, may be an extension of the 

 Amyzon shales. These in turn rest on a formation of hard lami- 

 nated beds, which contain an abundance of Catamites, which 

 doubtless belong to the Triassic or Jurassic period. The Truckee 

 beds are, like the true White River, overlaid by the Loup Fork, 

 and this in turn by heavy beds of basalt. 



The fauna of the Truckee presents some characters which dis- 

 tinguish it from that of the White River. These are, the absence 

 of Hycenodon t Leptictidce and Ischyromys, and most of the Meno- 

 dontidce, and the presence of several genera of Canidiz, Nimravidce 

 and Rodentia. Many genera, and apparently several species, were 

 common to the two epochs. 



This formation has now been studied in many widely-separated 

 localities in the region west of the Mississippi river. It was dis- 

 covered by Dr. Hayden, whose collections furnished the basis of 

 Dr. Leidy's determination in 1 85s. 2 It was next observed by 

 myself in Colorado in 1873, 3 and twenty-one species were deter- 

 mined ; and in the following year I identified the Santa Fe marls 

 of New Mexico, already observed by Dr. Hayden, with the same 



8 Bulletin of the U. S. Geol. 



