USE OF TERMS JURA AND JURA-TRIAS 387 



ment of the Como. The unconformabilities formed by these changes 

 are apparent, but have not been detected. 



The Jura and Jura-Trias 



Early writers * on Rocky Mountain geology called these stages Jurassic 

 without hesitation. In more recent years a fauna was discovered in 

 western Wyoming and eastern Idaho which was supposed to be a mix- 

 ture of Jurassic and Triassic types. In consequence the term Jura-Trias f 

 was introduced, and since that time it has been applied alike to the rocks 

 of the eastern Rocky mountains and to those farther west. There seems 

 to be no reasonable ground for continuing the name Jura-Trias for the 

 eastern Rocky mountains, for the Jurassic rocks are easily distinguished 

 from the " Red beds," which may be either Permian or Triassic. Litho- 

 logicalty they favor the Triassic and should be so considered unless a 

 fauna or flora should prove them otherwise. 



Suggestions as to Correlation 



The correlation of these stages with those of European Jurassic can 

 not be satisfactorily accomplished until more is known of the American 

 fauna. Professor Hyatt J has already suggested that the Rocky Moun- 

 tain Jurassic corresponds to the Upper and Middle Jurassic of England, 

 in which case the Como would equal the Oxfordian, and the Shirley the 

 Oolites. There can be no mistake in assigning the Como stage to the 

 Upper Jurassic, but it seems quite possible that it is more closely allied 

 to the Purbeckian than to the Oxfordian. 



The Shirley stage presents a series of complications which renders its 

 correlation with the foreign Jurassic an impossibility in the present con- 

 dition of our knowledge, and it seems quite possible that there is no 

 corresponding member in Europe. The invertebrate fauna is not exten- 

 sive, and there is a great scarcity of Cephalopods, which are the leading 

 horizon indicators of Europe. To review the invertebrate fauna, it seems 

 quite probable that this stage also should be assigned to the Upper 

 Jurassic. On the other hand, the vertebrate evidence is so conflicting 

 that one is at a loss to suggest a corresponding stage. For instance, two 

 plesiosaurs and some fishes have been discovered which have Liassic 



* See early writers, Hayden's reports of the United States Geological Survey of the Territories 

 from 1868 to 1876, inclusive ; also earlier papers by Meek & Hay den. The Forteith Parallel Survey 

 and all literature relating to this formation in the Rocky mountains prior to 1877. 



fSee United States Geological Survey of the Territories of Idaho and Montana, 1877, Hayden, pp. 

 359, 556, 559-561, 621-623, 625, 626, 628. 



| See Bull. Geol. Soc. Am., vol. 3, 1802, pp. 4u'.>, 410. 



