62 F. II. Knowlton — Jurassic Flora of Oregon. 



The intense folding, vulcanism and partial metamorphistn of 

 the Mariposa is held to be a .stron<!; indication of its greater 

 age as compared with the Knoxville, where there has been 

 comparatively little folding or other alteration ; yet even this 

 should apparently be accepted with a measure of caution, since 

 it is by no means clear that the various students who have 

 worked in this field have always given the same limits to the 

 Knoxville. Thus the Dillard area of Oregon seems to be more 

 or less of an exception. In this area the rocks have undergone 

 silicitication and veining similar to that of the Mariposa, and 

 on these lithologic as well as other grounds are regarded by 

 Londerbach as identical with the Franciscan of California, 

 which is almost certainly the approximate equivalent of the 

 Mariposa, if not indeed below it. However, from the presence 

 in the rocks of the Dillard area of two species of Aucella 

 {A. Fiocliii and A. crassicollis), Diller considers them refer- 

 able to the Myrtle (Knoxville). But since the shells occur in 

 calcareous pebbles in a conglomerate it is possible that they 

 were derived from an earlier Aucella-bearing horizon, though 

 it is but fair to state that Diller concludes that they may be 

 concretions rather than true rolled pebbles. 



Relations between Dothan and Mariposa. 

 In the Oregon areas here considered the Jurassic plant beds 

 are unconformably underlain in many places by rocks to 

 which the name Dothan formation has been given, and these 

 in turn have been tentatively correlated with the Franciscan of 

 California. The Dothan rocks, chiefly sandstones with some 

 slates, which have been more or less silicified and veined, are 

 characterized by the presence of Aucella Erringtoni, a species 

 found also in the Mariposa. The only plant thus far noted in 

 the Dothan was found on Catching Creek, about six miles 

 southwest of Riddles, and was identified as probably the upper 

 part of a leaf of Pterophyllum, such, for example, as F. mquale 

 (Brongn.) ISTath. Although this is so fragmentary as to be of 

 little value in fixing the age, it is nevertheless a suggestion 

 that if a flora could be developed in these beds, it would more 

 than likely bring out a similarity to that of the Oroville (Monte 

 de Oro) flora. 



The Line between the Jurassic and the Cretaceous. 



It has been held on structural and stratigraphic grounds, 

 that in this area the line between Jurassic and Cretaceous 

 should be drawn at the base of the Knoxville. However, the 

 paleobotanical evidence that has been presented in this paper 

 shows that, notwithstanding the unconformable relations exist- 

 ing between the Knoxville and underlying beds, this is not a 



