MESOZOIC OF SOUTHWESTERN OREGON 529 



commonly so badly weathered, that their detailed study, especially 

 that of the basic series, would require considerable time. Only 

 their more striking characteristics and more evident field groupings 

 are given here. 



The basic series. — The igneous rocks included under this heading- 

 are the most abundant, the most varied, and the most difficult to 

 study of any in the region. In the text of the Roseburg folio they 

 have been described under the title of metagabbro. In the Coos 

 Bay map they are incorrectly grouped with the Eocene basalts, and 

 given the same color and letter symbol, although in the text it is 

 stated that certain of the areas are "more highly altered than those 

 already noted, and are associated with the Myrtle formation. They 

 are probably of greater age than those found in the Pulaski formation 1 

 and are the product of eruptions occurring at the close of the Creta- 

 ceous." 2 In the text of the Port Orford folio they are separated into 

 two groups, and described as gabbros and basalts, respectively. 



Within the basic series are rocks that are petrographically different, 

 but are probably genetically related. On the other hand, there are 

 included rocks that are not closely related. The briefness of the 

 writer's study does not allow of a satisfactory genetic subgrouping or 

 a complete detailed description of these rocks, but certain general 

 characteristics and more obvious groupings may be given. 



The greenstones. — A very characteristic field type in the Dillard 

 series is rather fine-grained, frequently compact, the grain generally 

 not being discernible to the naked eye, and of a dull green color 

 weathering to brown. While sometimes producing prominent out- 

 crops, these rocks frequently weather down to a brownish earth, 

 resembling on superficial examination the weathering products of 

 the sandstones, and the boundaries of their areas then become fre- 

 quently difficult, or at least require considerable time, to determine; 

 indeed, small areas may easily be overlooked entirely. In texture 

 these rocks vary from compact — perhaps sometimes originally 

 glassy — to fine granular or fine diabasic. The normal or average 

 type may be called basaltic, consisting essentially of pyroxene and 



1 A division of the Eocene of the Coos Bay quadrangle. 



2 Cretaceous was there used as a synonym for Myrtle, and should now read Dillard, 

 as they were undoubtedly erupted before the deposition of the basal beds of the Myrtle 

 group. 



