392 



J. S. DILLER THE JURASSIC FLORA OF OREGON 



succession is A^ell illiTstrated on Mja-tle creek, Elk river, and Eogne river, 

 in Oregon, and locally in the Klamatli mountains, but most fi^Uy and 

 satisfactoriljf along the western side of the Sacramento valley, in Cali- 

 fornia. 



While it is possible that Aucella crassicollis may occur much farther 

 down in the series than 2,000 feet from the top of the Knoxville beds, it 

 has not yet been shown to have such earlier distribution, and until such 

 evidence appears we are constrained to regard the section of the western 

 side of the Sacramento valley as the standard. 



Local Silicification of the "Myrtle Formation" 

 "myrtle" generally unaltered 



In the Gold l^elt of the Sierra Nevada of California the Paleozoic 

 strata were deformed and metamorphosed to a considerable degree before 

 the deposition of the Mesozoic, but the great epoch of mineralization ap- 

 pears to have taken j^lace at the close of the Mariposa. This is strongly 

 indicated by the general fact that the Mariposa slates are often highly 

 altered and contain important ore bodies, while the nearest Knoxville 

 strata are imaltered. 



The greatest mass of Knoxville strata known lies along the western 

 side of the Sacramento valle}', in Tehama county, California, and 

 although locally veined with calcite is not perceptibly metamorphosed. 

 A similar mass, not so thick, but quite unaltered, lies in Cow Creek valley 

 and along ]\Iyrtle creek, in Douglas county, Oregon. 



EXCEPTION TO GENERAL RULE 



The Myrtle Creek mass noted above is limited on the northwest by 

 Dodson Mountain ridge, beyond which, in the Dillard region, an area of 

 the Knoxville occurs that is an exception to the general rule, for its rocks 

 are locally affected by a considerable degree of silicification. Attention 

 has been recently called to the rocks of this area by Mr Louderback, who 

 regards its silicihed rocks bearing quartz veins in variable abundance as 

 typical of the Franciscan of California. "While not questioning the litho- 

 logical comparisons which Doctor Louderback's familiarity with- the 

 Franciscan has enabled him to make, I have been especially interested in 

 the age of the rocks and the relation of the fossils to the silicification. 



CALCAREOUS CEMENT AND QUARTZ VEINS IN THE "MYRTLE" 



The cement in the sandstones and conglomerates of the Dillard area 

 varies greatly and often abruptly from- place to place. Where fossil shells 

 are abundant, the cement is calcareous; but where they are rare or absent, 

 especially in the sandstones, the cement may be siliceous and the rocks 



