MESOZOIC OF SOUTHWESTERN OREGON 54] 



The Dillard area extends southwest beyond the limits of the map, 

 into the vicinity of Olalla, and finally passes under the later rocks of 

 the Coast Range. On the north also it is limited by the Eocene 

 sediments and volcanics. In common with all the other pre-Tertiary 

 rocks of this and the Riddles quadrangle, the Dillard series and its 

 associated igneous rocks are bounded on the east by the lavas and 

 tuffs of the Cascade Range, which, in the same direction, are followed 

 by the lavas, etc., of the interior plateau region. The area under 

 consideration is the most northern exposure of the Dillard series 

 known. 



The structure of the Dillard area cannot be definitely stated, 

 except that the rocks have a general northeast-southwest strike, which, 

 however, is locally varied without limit. In the section D-D of the 

 Roseburg folio, across the strike of the series, it is represented as a 

 succession of folds — three synclines and two anticlines — the whole 

 forming a sort of synclinorium. The section C-C, six or seven 

 miles farther northeast, represents the structure as monoclinal with 

 a northwest dip. No suggestion is offered as to the manner in which 

 the one structure passes into the other. 



The Myrtle Creek area. — In contrast with the Dillard area, the 

 Myrtle Creek area is made up almost entirely of the members of the 

 Myrtle group. These occupy all of the area included in the Rose- 

 burg quadrangle except a small patch in the upper northeast corner 

 near the head of Bilger Creek. In that corner the characteristic 

 light gray, highly cemented, veined sandstones of the Dillard series 

 occur, with irregular and indefinite structure; and, nearby, the 

 regularly bedded, comparatively unaltered, fossiliferous shales and 

 other members of the Myrtle group. The actual contact between 

 the sedimentary members of the two groups was not observed, but 

 they were exposed within 100 yards of each other. 



The Myrtle Creek area has, in general, a simple structure. The 

 strata strike and the area extends in a northeast-southwest direction, 

 and the rocks have been folded into a simple syncline about three 

 miles across as now exposed. This synclinal structure extends 

 throughout the area that is included in the Roseburg quadrangle, 

 and continues to the southwest into the Riddles quadrangle. It 

 there lies on both sides of the Umpqua River, which it leaves in the 



