MESOZOIC OF SOUTHWESTERN OREGON 533 



The serpentine dikes cut into and through the members of the 

 basic series — both coarse-grained rocks and greenstones. This 

 relationship has been observed at a number of different places both 

 in the Roseburg area and in the coast quadrangles. 



The dacite-andesite group. — The rocks of the dacite-andesite 

 group are much less abundant than any of the above described 

 igneous series, but they occur at intervals on the Roseburg, Riddles, 

 and Port Orford quadrangles, and show but a comparatively small 

 range of characters throughout these areas. They present in general 

 a rather compact groundmass, which differs from the other fine- 

 grained rocks associated with the Dillard series in its usual pale 

 colors, light yellowish, slightly pink, or white, and in its superior 

 hardness. The most common type shows phenocrysts of quartz 

 corroded by the magma, and a soda-lime feldspar, usually more 

 abundant than the quartz. Ferromagnesian minerals are rare as 

 phenocrysts and of very subordinate development (usually hornblende) 

 in the groundmass. One common type has a fine, even grain, 

 appearing to the eye as if without porphyritic structure. In general 

 appearance it resembles quartzite. Under the microscope, however, 

 its relationship to the distinctly porphyritic types is readily recog- 

 nized. Analyses given in the text of the Port Orford folio show 

 about 70 to 75 per cent, silica, 4 to 7 per cent, soda, and 2 to 2\ per 

 cent, potash. Both chemically and in mineral composition they 

 belong with the dacites. 



A number of the specimens examined exhibit the same structures 

 and general appearance as described above, but with no visible 

 phenocrysts of quartz. In thin sections made of some of these no 

 quartz could be found either as phenocrysts or in the groundmass. 

 Although none of these were analyzed chemically, they may be referred 

 to the andesites. As they are closely related to the dacites in the 

 field, and show gradational forms carrying varying proportions of 

 quartz, the two mineralogical types are considered to be genetically 

 related and placed in a dacite-andesite group. 



The rocks of this group occur in small irregular bunches and in 

 elongated, dike-like masses in the Dillard series, and not uncommonly 

 within the serpentines which they appear to cut. They are described 

 and mapped in the Roseburg folio as "dacitic rocks," and in the 

 Port Orford folio as dacite-porphyry. 



