16 



I. THE DAKOTA EPOCH. 



The sandstones of this division are light-brown, buff, or white, moder- 

 ately soft or very hard, and varied with occasional conglomerates. They are 

 extensively developed on the Missouri, reaching 1,500 to 2,000 feet in depth. 

 They appear all along the eastern flank of the Rocky Mountains, and on the 

 west side of the Sierra Madre or San Juan. No vertebrate fossils have yet 

 been obtained from them. 



II. THE BENTON EPOCH. 



This formation embraces dark, lead-colored shales and clays, and is found 

 lying on the preceding in most regions where it occurs. It includes many 

 mollusks, some of which are identical with those of No. 3. Among these are 

 Ostrea congesta and Inoceramus prob/ematicus. Four vertebrate species have 

 been defined from the numerous remains discovered, viz : Lemma ? cuspidata, 

 a shark ; Pelecorapis varius, an ally of the flying-fish ; Apsopelix sauriformis, 

 a related fish ; and Hyposaurus vebbii, a gavial-like crocodile. 



III. — THE NIOBRARA EPOCH. 



The exposures of this formation have a wide area between the Carbonif- 

 erous and Lower Cretaceous beds of the east of the plains and the Rocky 

 Mountains, and in Texas and Eastern New Mexico. Vertebrate fossils are 

 very abundant, and I have received them from Kansas, Colorado, Texas, 1 and 

 New Me.vico. The following description is from the notes of my Kansas 

 expedition of 1871 : 



" The geology of the regions marked by this formation is quite simple. 

 The following description of the section along the line of the Kansas Pacific 

 Railroad will probably apply to similar sections north and south of it. The 

 formations referable to the Cretaceous period on this line are the Dakota, 

 Benton, and Niobrara groups, or Nos. 1, 2, and 3. According to Leconte, 2 

 at Salina, one hundred and eighty-five miles west of the State line of Missouri, 

 the rocks of the Dakota group constitute the bluffs, and continue to do so as 

 far as Fort Harker, thirty-three miles farther west. They are "a coarse, 



1 From this region from Dr. A. R. Roessler. 



-Notes on the Geology of the Survey for the Extension of the Union Pacific Road, Eastern Division, 

 from the Smoky Hill to the Rio Grande. By .Tohn L. Eeconte, M. D., Philadelphia, 1868. 



