184 T. W. STANTON 



undescribed. The strata have been almost equally divided into the 

 Enochkin formation below and the Naknek formation above. The 

 upper part of the Enochkin formation is characterized by a great 

 development of the ammonite genus Cadoceras, indicating the boreal 

 facies of the Callovian stage, while the Naknek formation contains 

 Cardioceras near the base and an abundance of Aucella with Lytoceras, 

 Phylloceras, etc., in the overlying beds. The fossils indicate that 

 the horizon of the Rocky Mountain Jurassic is near the boundary 

 between the Enochkin and Naknek formations. In other words this 

 Rocky Mountain epicontinental sea, which W. N. Logan has dis- 

 cussed,' was drained before the deposition of the Jurassic "Aucella 

 beds" which have such a great development in Alaska and farther 

 south on the Pacific coast as well as in Russia and in many areas of 

 the boreal region. Its fauna is clearly boreal, as has already been 

 stated, and there w'as marine connection either directly with the Arctic 

 Ocean, or, as the known distribution of the rocks makes more prob- 

 able, indirectly through the north Pacific somewhere between Van- 

 couver Island and Cook Inlet. There seems to have been no direct 

 connection with the contemporaneous sea of California w^hich had a 

 different, though imperfectly known, fauna more closely related to 

 middle European faunas. 



After the sea had retreated from the Rocky Mountains the boreal 

 Aucella fauna which occurs in the Naknek formation extended down 

 along the Pacific coast into Oregon and California where it charac- 

 terizes the Mariposa slate and equivalent formations, continuing 

 through a great thickness of strata to the top of the Jurassic and 

 passing without any striking change into the Lower Cretaceous. 



In Mexico no faunas are known that belong to the Middle Jurassic, 

 or to the Callovian and Oxfordian stages of the Upper Jurassic, but 

 the later stages, or at least the Kimmeridgian and Portlandian, are 

 well represented near Mazapil in the state of Zacatecas and in adja- 

 cent portions of neighboring states. Burckhardt who has recently 

 described the fauna^ states that it resembles the faunas of central 

 Europe and the Mediterranean but that it also contains forms that 

 show relationship with the Russian or boreal fauna and others that 



1 Jour, of Geol., Vol. VIII (1900), pp. 241-73. 



2 Institute Geologico de Mejico, Boletin No. 2J, 1906. 



