1887] Mesozoic and Cenozoic of North America, 447 
The Jurassic bed constantly overlies the Triassic along the 
flanks of all the Rocky Mountain ranges, consisting of clays, 
shales, marls, and cherty limestones. In Colorado it has, accord- 
ing to King, a thickness of seventy-five to two hundred and fifty 
feet. It grows thicker westward, reaching seven hundred feet 
on the west flanks of the Sierra Madre, in New Mexico, and, 
according to King, consists in Nevada of— 
Feet, 
Slates 
Limestone 1500 to 2000 
The forms of vertebrates found apparently together at this 
horizon are represented in Europe by genera of different sub- 
divisions of the Jurassic. Hence it has not been possible to 
refer the Rocky Mountain beds to any of the latter, and Marsh 
has therefore designated them as the A//antosaurus beds2* 
A series of deposits lies between the Triassic and Cretaceous 
formations in the Middle Atlantic States, which have been sup- 
posed from the palzobotany to be of Jurassic age by Tyson”, 
What division, if any, of the European series they represent has 
not yet been ascertained, but they are regarded by Mr. McGee 
as belonging at the summit of the system. He names them the 
Potomac formation in an unpublished memoir.” . 
CRETACIC SYSTEM. 
Characteristics.—Presence of Saurodont® and Actinochirous 
fishes; of Reptilia Eusuchia, Testudinata Protostegide, Pro- 
pleuridæ, and Adocide ; of Aves Odontornithes. Absence of 
Pisces Ginglymodi** and Halecomorphi; of Reptilia Chorist- 
dera, Dinosauria Opisthoccela. 
This,formation has great extent piä thickness in North Amer- 
ica, and it displays a number of divisions, which differ both 
lithologically and faunally. These are the 
ills, 
Comanche. 
THE ComaNncHE has been recently named and described by 
Hill.7- It consists principally of limestones of varying character 
