580 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



very thick and seemingly passes unbroken into marine deposits of Jurassic 

 age. Plants are said to be abundant. 



Jurassic Period 

 See plates 88 to 90 



Pacific region. — The unstable crustal condition along the Eocky Moun- 

 tain axis during the Triassic was changed in early Jurassic times into epei- 

 rogenic elevation, thus removing the late Triassic submergence from all 

 areas excepting that of the Cook Inlet and Selikorf region of Alaska, and 

 the states of California, Oregon, and Nevada. With these movements 

 there again appear in California marked faunal changes, for the boreal 

 migrations of late Triassic times have ceased, and faunas with Arietites 

 are now present, which Smith thinks may be South American invaders of 

 Mediterranean derivation. 



The Alaskan Lias submergence is continued into the Middle Jurassic of 

 the Enochkin formation and has a thickness of 1,600 feet. According to 

 Stanton and Martin, 227 the lower third of this series is marked by the 

 bivalves Inoceramus ambiguus, I. porrectus, I. eximius, and I. lucifer, 

 besides the ammonites Stephanoceras loganmm, 8. carlottense, Sphcero- 

 ceras oblatum, and 8. cepoides. Most of these shells also occur on the 

 Queen Charlotte islands. The upper two-thirds of the Enochkin are 

 marked by Cadoceras doroschini, C. schmidti, and C. worsessenskii. 



Along the Pacific border the Middle Jurassic is continued into the 

 Upper Jurassic. In the Cook Inlet country of Alaska the Enochkin per- 

 sists without break into the Naknek, having a thickness of 5,000 feet or 

 more (including some andesite). The guide fossil of this region is a 

 boreal mollusk, either identical with or close to Aucella pallasi. With it 

 are associated the boreal ammonites Cardioceras alternans and C. cordatus. 



At or near the close of the Middle Jurassic, or, according to Stanton 

 and Martin, during the early part of the Naknek, in the northern Great 

 Plains area, there appeared an inundation of wide extent — the Logan 

 sea — bringing in for a short time a North Pacific or boreal fauna distin- 

 guished by the ammonites Cardioceras cordiforme and Cadoceras, and the 

 cephalopod Belemnites densus. The fauna is not a large one and con- 

 sists mostly of bivalves, as Pseudomonotis curta, Astarte packardi, Pleu- 

 romya subcompressa, Tancredia bulbosa, T. magna, Goniomya montanen- 

 sis, Lima lata, Camptonectes bellistriatus, Grryplima calceola nebrascensis, 

 and Ostrea strigilecula; also the ichthysaur Baptanodon discus. In the 

 Great Plains region the formation has a thickness varying between 100 



227 Stanton and Martin : Bull, of the Geological Society of America, vol. 16, 1905. 



