SALIENT FEATURES OF THE GEOLOGY OF OREGON 91 



age of these igneous rocks, their exact geologic age is unknown, 

 they being presumably of late Jurassic age. 



Cretaceous. — This system is developed along the southwestern 

 flank of the Blue Mountains, eastern Oregon, and mainly within the 

 valleys of the Rogue, Umpqua, and Coquille rivers of southwestern 

 Oregon. The best-known localities within the former region are 

 Mitchell' and the vicinity of An tone in Wheeler County. Broad 

 exposures are found within the Port Orford, Coos Bay, and Rose- 

 burg quadrangles, and well-recognized areas in the vicinity of 

 Riddle, Medford, and Ashland." 



These rocks in part represent the Shasta- Chico series of Cali- 

 fornia yielding characteristic faunas and being lithologically similar 

 to that series. The Shasta includes the Myrtle of Diller, whose 

 most recent definition would include even the equivalent of the 

 lowermost Knoxville as yet unknown in Oregon.^ 



The lower Myrtle, or the Knoxville of Oregon, as found in the 

 above-named quadrangles, is composed of shales, sandstones, 

 conglomerates, and lenses of limestones, all of which are in places 

 locally metamorphosed and intruded by basic rocks. Dark-blue 

 shales so typical of the Knoxville are known in several lower Myrtle 

 areas in southwestern Oregon as well as at Mitchell in eastern 

 Oregon. This latter locality has not as yet yielded determinable 

 fossils, so its correlation with the Knoxville is only tentative. 

 Heavy Myrtle conglomerates resting upon a basement complex 

 representing successively higher horizons indicate a transgressing 

 sea, which condition apparently continued until the close of the 

 period. 



The upper Myrtle, containing a Horsetown fauna, is well devel- 

 oped in the Myrtle Creek syncline of the Roseburg and Riddle 

 quadrangles. It is not known to occur east of the Cascades. 

 These rocks are somewhat more variable lithologically than the 

 Knoxville, probably being more conglomeratic. 



The Chico group along Rogue River consists of several thousand 

 feet of sediments formed in a transgressing sea, which perhaps 



1 J. C. Merriam, Univ. Cal. Publ. Bull. Dept. Geol., II, 280. 



2 F. M. x\nderson, Proc. Cal. Acad. Sci., Series 3, II, 1-154- 



3 B. Willis, U.S. Geol. Surv., Prof. Paper No. 71, p. 618. 



