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GEORGE DAVIS LOUDERBACK 



they form very prominent exposures, and are likely to appear more- 

 abundant than they really are. 



These cherts are of a prevailing red color, but may be perfectly 

 white or green, brown, yellow, or gray — sometimes variegated. 

 They are generally distinctly stratified, the silicious layers varying 

 from an inch or two to a foot or more — two to four inches being the 

 more common values. These are separated by a peculiar, generally 

 dark red ferruginous shale, varying in thickness from little more 

 than a film between the chert layers up to an inch or more. Several 

 hundred alternations of cherty and shaly layers may occur in a single 

 exposure. These layers are sometimes badly crumpled, and not 

 infrequently, in the smaller masses, the stratification may be almost 

 or entirely obliterated. 



Small, clear chalcedonic spots are frequent indications of original 

 radiolaria, and sometimes more or less of the structure or ornamen- 

 tation of the original test is preserved. The bulk of the silicious 

 layers are, however, apparently recrystallized and are very fine- 

 grained (microcrystalline) quartz aggregates. These layers are also 

 traversed in all directions by minute quartz veins. 



The cherts occur either as lenses in the sandstones or as inclusions 

 in the basic igneous rocks which cut the lower series. 



Limestone. — The only limestone noted occurs in a series of lenses 

 described in the text of the Roseburg folio as the Whitsett limestone 

 lentils. These are in general gray and massive, showing more or 

 less the effects of crushing, and, while some recrystallization is pres- 

 ent, there has not been any noteworthy development of a phanero- 

 crystalline-granular structure in the larger lenses at least. Some 

 of the smaller areas appear to be more altered and to have developed 

 a fine-grained to compact crystalline structure. The more altered 

 ones show an abundance of calcite veinlets up to about two inches 

 in thickness. They are also traversed irregularly by a secondary 

 pink chert which shows the effects of fracturing since its formation. 

 The matrix is sometimes colored variously, yellow, brown, etc. ; the 

 veins are generally white. In some parts oolitic structures were 

 observed, and in some the more or less abundant remains of micro- 

 scopic organisms — foraminifera in the calcareous parts, and, in the 

 associated chert, apparently radiolaria. 



