SUMMARY. 373 



Concerning the granite and basalt there has scarcely been a question. 

 They are eminently characteristic occurrences. The nietamorphic diorite 

 sometimes resembles eruptive diorite, and has been taken both for diorite 

 and granite; usually it bears some resemblance to augite-andesite or basalt, 

 and has been determined microscopically as an unusual variety of the 

 latter rock. It is composed essentially of oligoclase and hornblende. The 

 hornblende was originally colorless, but through some change (perhaps 

 absorption of water) it is in large part converted into an intensely green 

 variety. The hornblende polarizes in unusually intense colors. 



The quartz-porphyry underlies both hornblende-andesite and diabase. 

 The microscope, Thoulet's method of separation, and analysis, show that 

 the predominant feldspar is orthoclase. It is characterized by the associa- 

 tion of liquid and glass inclusions usual in quartz-porphyry, to which also 

 the groundmass corresponds. In one locality, near the Red Jacket, the 

 quartz is nearly suppressed, and the rock is excessively fine-grained. It is 

 a felsitic modification of the ordinary variety. This rock, which Baron v. 

 Richthofen determined correctly, has since been called quartz-propylite, 

 dacite, and in its felsitic modification rhyolite. Most of the quartz-porphyry 

 is greatly decomposed. 



The eruptive diorite is sometimes granular, sometimes porphyritic. In 

 the porphyritic diorite mica frequently predominates over hornblende. 

 Quartz is irregularly disseminated through the rock. In the granular dio- 

 rite the hornblende is sometimes green and fibrous, sometimes brown and 

 solid. In some cases it can be shown that the latter variety of hornblende 

 is altered to the former, and possibly this is ordinarily the case. Augite is 

 not uncommon, and a part of the fibrous green hornblende is very likely 

 uralite, but in the granular rock the outlines of the crystalline grains are 

 rarely sufficiently regular to determine this point. In the porphyritic dio- 

 rites the fresh hornblende is always brown. Even in this latter variety of 

 the diorites well-developed feldspars are rare. The porphyritic diorites have 

 for the most part been regarded as propylite, and some occurrences of the 

 granular rock have been classed in the same way. Some of the fresher 

 porphyritic diorites have been mistaken for andesites, the resemblance to 

 which is occasionally strong. 



