LOWER CRETACEOUS. 619 



THESIS OF THIS PAPER. 



At variance with, the conclusions above set forth, it will be shown in the present paper 

 that the so-called "Jurassic flora of Oregon" is everywhere of true Jurassic age, that it is prac- 

 tically never "• found in association with the acknowledged Cretaceous flora, and finally, that 

 the line between the Jurassic and the Cretaceous of the region is to be drawn through the upper 

 part of the KnoxviUe and not at its base. 



SUMMARY. 



1. The Lower Mesozoic strata of Oregon and California have afforded two floras, one of 

 which is Cretaceous and the other Jurassic. These two floras, although sometimes occurring 

 practically in the same section, have not been found commingling. 



2. The Cretaceous flora ranges from the extreme upper part of the Knoxville formation 

 through the Horsetown formation. It embraces about 60 species of plants and is regarded as 

 being of Lower Cretaceous (Neocomian) age. It finds its closest affinity with the Kootenai of 

 the Interior region, the Trinity of Texas, and the lower Potomac of the Atlantic coast. 



3. Associated with the Lower Cretaceous flora is an invertebrate fauna regarded as being 

 •of Neocomian age, the plants and invertebrates thus being in agreement. 



4. The Jurassic flora, which has been called the "Jurassic flora of Oregon," ranges from 

 beds which have been referred to the Mariposa formation through the major portion of the 

 Knoxville formation. It includes 100 species and finds its close affinity with the Lower Oolite 

 floras of known position in other parts of the world. It is beyond question a true Jurassic flora. 



5. Associated with the Jurassic flora is a meager, often poorly preserved invertebrate 

 fauna of only seven species, including the two supposedly characteristic species of Aucella 

 {A. crassicollis and A. piochii). This fauna, with the exception of Aucella piocTiii, is the same 

 as that found in association with the Lower Cretaceous flora, and it is on account of this asso- 

 ciation that the Jurassic plant beds have been referred by some invertebrate paleontologists to 

 the Cretaceous. 



6. The age of the beds containing the "Jurassic flora of Oregon" thus hinges on the relative 

 ;strength of the evidence afforded by the flora as compared with that of the associated fauna. 

 It has been shown in this paper that of a total of 100 species of plants, 47 species are loiown 

 also from Imown Jurassic of other, often widely separated parts of the world. Only one of 

 these 100 species has been found in the Lower Cretaceous beds of the region. The total Knox- 

 viUe fauna comprises 77 forms of invertebrates, only 7 named species of which have been found 

 associated with the Jurassic plants. Of these 7 species of invertebrates, only a single species 

 has been found outside the limits of the Oregon-Cahfornia area, and it has been further shown 

 that the invertebrate paleontologists are not in accord among themselves as to the interpretation 

 to be given the age determination of the fauna. The conclusion is reached that the plants, 

 being thoroughly consistent, afford the better criteria, and the beds are regarded as unquestion- 

 ably of Jurassic age. 



7. From the paleobotanical evidence which has been presented it foUows that in this 

 portion of the Pacific coast region the fine between the Jurassic and Cretaceous is to be drawn 

 through the upper part of the KnoxviUe formation, and not at its base. This line is fixed by 

 the upper limit of the Jurassic flora. 



In 1895 Stanton discussed the relation of the Knoxville fauna to the Volgian 

 faunas of Russia, and after citing differences of opinion among the Russian geolo- 

 gists as to the Jurassic or Cretaceous age of the Volgian, he wrote :"^^ 



It is evident, then, that even if it were possible to refer the KnoxviUe to definite horizons 

 of the Russian Aucella beds its age might still be questioned. One important fact that should 



o The only possible exceptions are a scrap very doubtfully identified from the area near Riddles, Greg., and a 

 conifer of no stratigraphic value from California. 



