MESOZOIC OF SOUTHWESTERN OREGON 519 



Orford areas on the west. Within this region the Coast Range is 

 made up almost entirely of Eocene rocks, and is, therefore, not of 

 particular interest in the present investigation; but in the country 

 both east and west of this range the Eocene sediments have been 

 largely stripped off, and considerable areas of the pre-Tertiary rocks 

 exposed. These in general have a trend from northeast to south- 

 west, cutting obliquely across the Roseburg quadrangle, passing 

 under the Eocene sediments of the Coast Range, emerging with a 

 similar trend on the west side of the range, and continuing to the 

 ocean. The writer's studies were practically confined to the south- 

 ern parts of the two northern quadrangles and the northern parts 

 of the two southern quadrangles. 



PRESENT KNOWLEDGE OF THE MESOZOIC OF THIS REGION 

 THE MYRTLE FORMATION 



Diller has included most of the Mesozoic of this region in the 

 "Myrtle formation." This name was originally given by him to 

 the Mesozoic of the Roseburg quadrangle (published in 1898), 

 because of the fact that it is most typically developed along Myrtle 

 Creek in the southern part of that area, and because in that vicinity 

 was found the greatest range of fossils. The lower part of the 

 Myrtle formation at that locality is stated to be characterized by 

 Aucella piochi and A. crassicollis, and to correspond, therefore, to 

 the Knoxville (a part of the Lower Cretaceous) formation of Cali- 

 fornia. The upper part is said to be characterized by Pecten oper- 

 culijormis, Trlgonia aquicostata, and other forms of diagnostic value, 

 indicating that it is of the same geological horizon as the Horsetown 

 beds of California (Lower Cretaceous), including the uppermost 

 horizons immediately below the base of the Chico. 



Certain rocks lying southwest of the limits of the Roseburg quad- 

 rangle are provisionally referred to the Jurassic, with the statement 

 that they may possibly extend onto the quadrangle ; and some cherts 

 occurring within the quadrangle are also made doubtfully Jurassic; 

 but these are both distinctly separated from the Myrtle formation 

 and discussed under a different heading. Moreover, a considerable 

 gap is noted between the Myrtle and the Umpqua (Eocene) forma- 

 tions, and is said to correspond to the Chico (Upper Cretaceous) of 

 California and other parts of Oregon. 



