Rogue, Jr. — Geology and Structure of Volcanic Rocks. 223 



The rock varies considerably in appearance from place to 

 place, depending upon its degree of silicification and schis- 

 tosity. Much of the massive tuff is highly siliceous, varies 

 in color from cream through gray to black, and breaks with a 

 conchoidal fracture into chips with keen, translucent edges. 

 The outcrops are badly jointed and emit a metallic sound 

 when struck with steel. This type resembles flint or chert 

 and is locally called "gunflint" Another phase of the rock 

 is less dense and not so siliceous ; it is usually light gray in 

 color and appears very finely granular. Still another phase is 

 dark green and represents an arenaceous phase of the tnff. 

 Much of the line tuff has suffered a variable degree of mash- 

 ing, so that all gradations are found from the massive rocks 

 into sericite schists. 



The microscopic character and transition into coarse tuff 

 establishes the volcanic origin of the fine tuff. It is thought 

 to represent a volcanic ash of rhyolitic to dacitic character, 

 which has been indurated, silicified, and altered. 



Acid coarse tuff. — The acid coarse tuff occurs associated 

 with the acid fine tuff in northeast trending belts, separated 

 from each other by bands of slate country. The rock is also 

 somewhat extensively distributed as narrow intercalations and 

 lenses within the slate belts. 



In common with most of the rocks of the district, the coarse 

 tuff is found in all gradations from a massive to a highly 

 schistose condition. Most of the outcrops reveal their frag- 

 mental nature on fresh fracture, but with increasing difficulty 

 in proportion as the rock is more severely mashed. The out- 

 crops are abundant and prominent ; well rounded when mas- 

 sive, and narrow and elongated when schistose. The weathered 

 surface is characteristically bumpy, due to the superior resist- 

 ance of the fragments, and has a yellowish or grayish color. 

 On fresh break, the rock is seen to be composed of a dense, 

 dark colored groundmass, containing broken crystals of feld- 

 spar and a variable number of small, angular rock fragments. 

 The latter are usually one-half inch or less in diameter and 

 represent several different kinds of rocks. Most abundant are 

 fragments of a dense, light colored, siliceous rock ; but pieces 

 of slate, sometimes showing bedding planes, and of a dark 

 colored, basic rock are not uncommon. 



Along the northwestern edge of the district occurs a broad 

 belt of sericite schists. These are light colored, extremely 

 fissile rocks, breaking into thin sheets which are smooth and 

 soapy to the feel and are quite soft. Upon weathering they 

 take on the most diverse and brilliant colors, especially near 

 mineralized zones. Associated with these occur outcrops 

 which have not been so badly mashed but that they show their 



