60 F. II. KnowUon — Jurassic Flora of Oregon. 



A choice must be made between tlie evidence afforded by the 

 two species of Aucella, since tliese are regarded as the most 

 important and characteristic of the invertebrates. If Attcella 

 crassicoUis is selected, then tliis portion of tlje Myrtle will be 

 upper Knoxville ; if Attcella Plochii is followed, this portion 

 may be lower Knoxville. In this connection it is to be 

 remembered that in certain sections of the Myrtle — such, for 

 instance, as tliat at Elk River — Aucella crassicoUis occurs 

 above, commino-led with, as well as below, the Jurassic plants, 

 while here or in nearby areas Aucella Piochii is found also 

 below, with and above the plants. 



In view of these conflicting alternatives it seems to the 

 writer that the plants, being thoroughly consistent, afford the 

 better criteria, and on their evidence the earlier portion of the 

 Myrtle is regarded as representing the Jurassic portion of the 

 Knoxville as a whole. If this be true, and it is believed to be, 

 then Attcella crassicoUis ranged from near the base of the 

 Knoxville (Myrtle) to near its top. 



As affording a possible confirmation of the position above 

 assigned the Myrtle on the basis of the plants, a word may be 

 said regarding a comparison of the depositional conditions 

 under which the Myrtle and Knoxville were laid down. It 

 needs but a glance at the geological folios involving the areas 

 covered by the Myrtle formation (Port Orford, Coos Bay and 

 Roseburg quadrangles) to show that during Myrtle time there 

 was a broad bay, or estuary, opening on the west to the Pacific 

 ocean, and extending northeast as far at least as the middle of 

 the Roseburg quadrangle. In the Sacramento valley in Knox- 

 ville time there was similarly a bay opening on the south and 

 extending northward between the Coast Range and the Sierra 

 Nevada. In both areas there was a progressive subsidence and 

 decided overlap. In the Myrtle area this extended northeast 

 and was most marked in the Riddles area. In the Sacramento 

 Yalley it of course proceeded northward, first the Knoxville, 

 then Horsetown, and finally Chico, resting successively on older 

 rocks. So far as known to the writer there is no a priori 

 reason why these two bays may not have been practically 

 synchronous. It will probably never be possible to settle this 

 point stratigraphically, since it is highly improbable that these 

 were at first, if indeed ever, connected on the present land 

 mass. 



That the Myrtle formation, as thus far made known, corre- 

 sponds only to the upper part of the Knoxville, while the still 

 earlier lower portions are concealed beneath the sea, or may 

 yet be found in some unexplored portion of Oregon, is to beg 

 the question. 



