LATER MESOZOIC INVERTEBRATE FAUNAS 183 



the northwest corner of Colorado, part of Idaho, and a considerable 

 area in British America. The fauna of this Rocky Mountain Jurassic 

 sea is characterized by Cardioceras cordijorme, Cadoceras, Belem- 

 nites densus, and a rather varied though not large fauna consisting 

 mostly of bivalves. It has long been recognized by Neumayr and 

 others to be of boreal type and hence as indicating a connection either 

 direct or indirect with the Arctic region. The fauna shows some 

 local variations, usually associated with variations in the character 

 of the sediments; but it appears to be essentially a unit throughout 

 the entire area. It is believed that the deposits were all formed in 

 one basin and within a comparatively brief period. Their maximum 

 thickness is usually only a few hundred feet. 



As there are no other marine Jurassic formations in the region and 

 the section is known to be incomplete it is necessary to go to other 

 areas where similar faunas occur to determine the exact position of 

 this one in the general column. In the Upper Jurassic the following 

 stages are recognized by De Lapparent who gives them universal 

 application : 



Purbeckian 



Portlandian , „ 



Isononian 



Kimmeridgian 



Sequanian 



Oxfordian 



Callovian 



The Jurassic of the Rocky Mountain region, as far as can be deter 

 mined from the fauna, represents the Oxfordian and perhaps the 

 Callovian in whole or in part. That is, it is the lower part of 

 the Upper Jurassic. In a large part of its area it rests on the Car- 

 boniferous, and the youngest marine fauna found beneath it any- 

 where is in the Lower Trias, while the oldest succeeding marine fauna 

 is Upper Cretaceous. It is obvious, therefore, that neither the ances- 

 tors nor the descendants of its species are found in the same area, but 

 fortunately its stratigraphic position is fairly well determined in the 

 much fuller Alaskan section. On the shores of Cook Inlet the Middle 

 and Upper Jurassic are represented by about 10,000 feet of strata 

 with at least three distinct marine faunas' which are largely still 



' Stanton and Martin, "Mesozoic Section on Cook Inlet and the Alaskan Penin- 

 sula," Geol. Soc. Am. Bull., Vol. XVI, pp. 391-410. 



