I''. II. Knowlton —Jurassic Flora of Oregon. 63 



natural line of delimination. According to the plants the 

 major portion of the great Knoxville section was deposited in 

 Jui'assic time, and the line should be drawn through the upper 

 Knoxville at the highest point (about 2000 rb feet below the 

 summit) at which the Jurassic flora occurs. This point corre- 

 sponds very nearly with the upper limit of the range of Aucella 

 Piochii, which is acknowledged to be of Jurassic type, and, as 

 already shown, extends down into the horizon of the Mariposa. 

 It is true that this places the line at a point where there is 

 no evidence of a break in the sedimentation, but it is a very 

 decided paleontological break, and is exactly similar in charac- 

 ter to that relied upon to separate Knoxville from Horsetown 

 in the same section. Although the Knoxville is of such great 

 thickness (20,000 feet), it is not probable that it required a 

 very long time, geologically speaking, for its deposition. 



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 occur- 

 ring practically in the same section, have not been found com- 

 mingling. 



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 inverte- 

 brate fauna regarded as being of Neocomian age, the plants 

 and invertebrates thus being in agreemeut. 



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. crassi- 

 collis and A. Piochii). This fauna, with the exception of 

 Aucella Piochii., 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. 



