MESOZOIC OF SOUTHWESTERN OREGON 531 



b) A group corresponding in range to the greenstones described 

 above. They include granular and coarse diabasic rocks made up 

 essentially of pyroxene and a moderately basic soda-lime feldspar, 

 which may perhaps be taken as the average or normal type. The 

 pyroxene is in some forms replaced by hornblende, giving rise to 

 dioritic rocks. These are apparently, sometimes at least, differentia- 

 tion products, and they ma,y sometimes be due to secondary alteration 

 of the original pyroxenes, as indicated by the term "metagabbro" 

 used in the Roseburg folio. The limit of variation in this group — 

 considered as a genetic group — has not been determined. It may, 

 and probably does, include more acid and more basic members than 

 above described. 



This group of coarse-grained rocks may be genetically related to 

 the greenstones already described, and it parallels them very closely 

 in mineral composition, and also in bulk chemical constitution, as 

 shown by the analyses given in the text of the Port Orford folio. 

 Types of intermediate grain also exist and may be considered as 

 physical gradations between the coarse-grained and the greenstone 

 series. 



The members of this group are, as far as observed, all intrusive 

 in the Dillard series. 



c) A third group, which is probably genetically distinct from the 

 one just described, may be called the quartz-diorite-diorite group. 

 These rocks are granitic in habit and structure, and have as their 

 essential constituents a more acid soda-lime feldspar, hornblende, 

 commonly quartz, and sometimes biotite. Chemically, according 

 to the analyses given for the Port Orford material, they appear to 

 vary from perhaps 50 to 60 per cent, in silica, to be rather low in lime 

 and magnesia (from about 4 to 6 per cent, and 2 to 3 per cent, respec- 

 tively), and high in alkalies (about 7 per cent, or over). The abun- 

 dance of original quartz in some of the sections studied would indicate 

 that the maximum silica percentage is above that given in the analyses, 

 and the rocks are very decidedly quartz-diorites — the quartz some- 

 times making up a third or more of the section. They are classed as. 

 acid types of the gabbro group in the text of the Port Orford folio, 

 but in habit, texture, mineral, and chemical composition appear to 

 be more closely related to the diorites and quartz-diorites which are 



