THE JURASSIC FAUNA. 269 



nous ; in the Niobrara, light- colored limestones predominate over the shaly 

 members, becoming chalks in the deeper portions of the seas. The Fort 

 Pierre is a great thickness of gray shales mostly argillaceous, while in the 

 Fox Hills the shales become more arenaceous and pass into sandstones at 

 the top of the formation. The Laramie is mainly sandstone in the enclosed 

 sea-basins near large land-masses, with an increasing admixture of shales 

 as the distance from these land-masses increases. 



An abundant and characteristic verterbrate fauna has been discovered 

 in the Jurassic beds at Como lake, in Wyoming, and at Canon City and 

 Morrison, in Colorado ; a somewhat meager fresh-water molluscan fauna is 

 associated with this in the two former localities, and some of the same forms 

 occur at a corresponding horizon in the Elk mountains of Colorado. They 

 are also reported from the Black Hills of Dakota and somewhat doubtfully 

 from the Green River basin of Wyoming. 



The Dakota formation carries an abundant flora which includes many de- 

 ciduous plants, but in the Rocky Mountain region no marine forms have 

 yet been found in it. The faunse of the other horizons of the Cretaceous up 

 to the Fox Hills are all marine, and in the Rocky Mountain region the change 

 from the marine forms in this horizon to brackish-water forms in the Lara- 

 mie is most marked and distinct. 



Jurassic Land. 



The more detailed and local effects of the Jurassic movement upon the 

 various land ai'eas under discussion were, so far as present facts afford any 

 indication, somewhat as follows : 



Colorado Island. — The general outline of Colorado island as determined in 

 early Palaeozoic time had thus far not been essentially changed. A general 

 encroachment of the ocean upon its shores had been in progress, whose effects 

 were more marked in the shallow bay-like depressions at its northern 

 and southern extremities than along its steeper east and west shore-lines. 

 The present areas of the North and Middle parks then formed a single 

 depression, the present dividing line between them having been formed in 

 post-Cretaceous times. North park had already been invaded by ocean sedi- 

 ments, and after the Jurassic movement further subsidence took place, so 

 that the sea extended through the Middle park connecting with the waters 

 occupying South park, and also across the Gore mountains westward to the 

 Colorado plateau waters. 



The relative distribution of the marine and fresh-water Jura is as yet but 

 imperfectly known. To the west of the Laramie plains, throughout the 

 Uinta and Wasatch regions and in eastern Idaho, the marine Jura is well 

 developed, but as yet no fresh-water beds have been recognized ; while at the 

 Como lake anticlinal both marine and fresh-water Jura are found. 



