412 EW. SLAN TON 
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 was 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 which 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 
connect it with the Jurassic of the South American Cordillera. He 
t Jour. of Geol., Vol. VIII (1900), pp. 241-73. 
2 Instituto Geoldgico de Méjico, Boletin No. 23, 1906. 
