Diller — Mesozoic Sediments of Southwestern Oregon. 413 



that the formations mapped as Myrtle senso stricto may be 

 separated into two chief groups or series (upper and lower), 

 each representing quite an extent of geological history, and 

 separated from each other by a distinct interval during which 

 there were intrusions of igneous rocks and a period of con- 

 siderable erosion." 



Lower series. — The lower series, according to Louderback, 

 is composed principally of gray sandstone so firmly lithified by 

 siliceous cement that when the rock is broken the fresh fracture 

 surface is smooth, traversing the grains. Besides this charac- 

 teristic sandstone the lower series contains some shale, occa- 

 sionally slaty, as well as foraminiferal limestone and radiolarian 

 chert, the last being very characteristic. For the lower series 

 Louderback proposes the name " Dillard", after a village situ- 

 ated on the railroad and south fork of the Umpqua River in the 

 midst of the largest area of this series in the Roseburg quad- 

 rangle. 



Upper group. — Louderback states that in the upper group 

 the most abundant and characteristic rock type is shale, then 

 sandstone and conglomerate. No trace of radiolarian cherts 

 or cherts of any type occurs in this group, nor do foraminiferal 

 limestones corresponding to the Whitsett lenses, although there 

 are calcareous shales. For this division Louderback proposes 

 to retain the name Myrtle group. 



Comparison. — Comparing his Dillard series and the Myrtle 

 group, Louderback remarks that their most striking general 

 difference is the markedly inferior lithification of the Myrtle 

 group, in which the sandstones (p. 525) " are commonly brown, 

 sometimes buff or greenish, and never show the peculiar gray 

 compact facies so often seen in the sandstones of the lower 

 series." In describing (p. 343) the strong contrast between the 

 close-lying areas of Myrtle and Dillard he points out that the 

 change from one to the other is abrupt and not gradational. 

 The only locality referred to where this contrast may be seen 

 is a few miles southwest of Dodson Mountain, but unfortu- 

 nately for that locality the two contrasted series are separated 

 by a belt of serpentine. The general impression conveyed in 

 Mr. Louderback's lengthy paper is that the two series are " well 

 defined and distinctly separable" lithologically and readily 

 discriminated in the field, epecially by one who is familar with 

 the Franciscan of California. 



Dillard Area of Dillard Series. 



Defined. — The largest area of the Myrtle formation out- 

 lined on the map of the Roseburg quadrangle centers about 

 the village of Dillard, where it crosses the Umpqua River and 

 is conveniently referred to by both Louderback and the writer 



