STRATIGRAPHIC SUMMARY. 651 



IX. The Jurassic System. 



The Jurassic, like the Triassic, is sparingly represented by 

 marine deposits in North America, these being confined chiefly to 

 the Pacific region of the United States and to Mexico. The Euro- 

 pean classification is adopted for these marine beds, the equivalency 

 being as follows : 



UPPER JURASSIC. 



TiTHONiAN — comprising 



Purbeckian, represented by the non-marine Como 

 beds of Wyoming, and (?) the Morrison beds of 

 the Front Range, though these are sometimes 

 classed as Comanchic. 

 Portlandian, represented by the upper 25 meters of 

 the Mazapil section, Mexico, and probably by the 

 Shirley beds of Wyoming and the Sundance of the 

 Black Hills, etc. The Upper Jurassic of Van- 

 couver Island and Arctic America also seems to 

 belong here. With the Tithonian as a v^hole is 

 placed also the Malone series of western Texas 

 and probably the Belemnites and Pentremites beds 

 of the Aspen Mountain, Idaho, and the upper part 

 of the Gold Belt slates (Mariposa formation) of 

 California, and some of the Upper Jurassic beds 

 of San Luis Potosi and other localities in Mexico. 

 Kimeridgian: represented by the middle part (60 

 meters) of the Mazapil section of Mexico, by the lower 

 part of the Jurassic beds of San Luis Potosi, and by a 

 part of the Mariposa formation, or Gold Belt slates of 

 California. 

 Sequanian and Corallian : represented by the Hinch- 

 man tuff of Shasta County, California, and the lower 

 beds of the Mazapil section of Mexico. 

 Oxfordian : representation not definitely known. 



MIDDLE JURASSIC. 



Callovian : represented by the Bic knell sandstone of 

 Shasta County, California. 



