408 Diller — Mesozoic Sediments of Southwestern Oregon. 



veinlets of quartz. Many of the sandstones, however, are less 

 firmly lithiiied and break with a rough surface due to the fact 

 that the grains are stronger than the cement and the fracture 

 passes between them instead of through them. The thin 

 beds of conglomerate are made up largely of siliceous pebbles. 

 The chert is of various colors, gray to red. It is sometimes 

 massive but more frequently banded with thin films of brown 

 shale. 



Occurrence. — The Dothan formation occupies a broad belt 

 running southwest from Cow Creek across Rogue River and 

 the Illinois, reaching the coast near the southern border of the 

 state. The larger area of Jurassic in southwest Oregon, as 

 outlined on the Geological Map of North America, is chiefly 

 Dothan. It lies northwest of the Galice area, from which it is 

 separated generally by a belt of volcanic rocks. The most 

 accessible and the best cross section of the series is on Cow 

 Creek and West Fork, where it may be seen along the railroad 

 in continuous exposures for nearly a dozen miles. 



The section begins nearly 4 miles southeast of Dothan, 

 where the rocks are mainly sandstone with less shale and a few 

 thin beds of siliceous conglomerate. Sandstones occasionally 

 massive attain a thickness of 100 feet, but most of the beds 

 are much thinner and with the shales clearly mark the position 

 of the strata dipping to the southeast beneath the sheets of 

 lava which limit their outcrop in that direction. Near the 

 border lava flows and tuffs occasional masses of chert are 

 found interbedded with the other sedimentary rocks. 



Farther down Cow Creek about Dothan and beyond on the 

 main stream to the limit of the Dothan formation near Nichols 

 Station, the proportion of shales is somewhat increased so that 

 in some places shale is more abundant than sandstone. In the 

 same portion of the section on West Fork, sandstones are de- 

 cidedly the most abundant. The beds range from 2 to 30 feet 

 in thickness and the stratification is conspicuous. As the north- 

 west side of the area is approached, the attitude of the strata 

 becomes more variable. Dips to the northwest become more 

 frequent though the general clip may remain to the southeast. 

 Quartz veinlets are abundant anywhere in cross fractures but 

 are never conspicuous. Those of calcite become more abun- 

 dant to the northwest near the Cretaceous, and lie mainly in 

 the plane of slaty cleavage as well as in the fault planes which 

 cut the earlier veins of quartz. 



Rogue River between Howard and Mule Creek, for a dis- 

 tance of over a dozen miles, has cut a rugged canyon directly 

 across the Dothan series and exposed an almost continuous 

 section. The principal difference between the Cow Creek and 

 Rogue River sections of the series is the presence of a large 



