R. Arnold — Hocks from the Sawtooth Range. 11 



films, probably of manganese oxide,* on the cleavage surfaces. 

 A small fragment of a fossil resembling Dentalium was 

 noticed in one specimen of slate and indicates the marine 

 origin of the formation. 



Metamorphosed Sedimentary Hocks. 



The metamorphosed sedimentary rocks consist of garnetif- 

 erous amphibolite schist, black schist, chert and jasper. They 

 are confined to narrow zones adjacent to the igneous intru- 

 sions, and are probably of contact origin. 



The garnetiferous amphibolite schist occurs intermittently 

 near the igneous dikes and is believed to represent a more 

 advanced stage of metamorphism than the black schists more 

 commonly associated with the igneous rocks of the region. 

 The garnetiferous rock is light greenish to drab in color, shows 

 the planes of schistosity distinctly and fractures with an 

 undulating surface parallel with the cleavage and with knife- 

 like edges in other directions. The specimen examined 

 contains numerous small pyrite crystals, mostly arranged in 

 thin layers parallel with the planes of schistosity ; small gar- 

 nets, though present, are not a common constituent of the 

 schist. Judging by the general appearance of the specimen, 

 the metamorphism of the rock was only partially completed. 



One wall of some of the mineralized veins of the region 

 consists of hardened bluish black schist, usually about 10 feet 

 in thickness, which has been only partially metamorphosed 

 and which grades to black slate and shale in a direction away 

 from the igneous contact. This schist is fine-grained, and 

 exhibits irregular crinkled cleavage faces. It fractures with 

 sharp edges across the planes of cleavage. In thin sections it 

 is seen to be composed of lens-shaped aggregations of quartz 

 surrounded by small parallel stringers of opacite. Mineraliza- 

 tion with pyrite often takes place in thin bands parallel with 

 the cleavage, especially near the contact veins. 



The only specimens of chert received came from the Black 

 Trail claim, about 2 miles west of Mt. Skokomish, w^here it 

 forms the wall of a quartz vein. It is very hard and fine- 

 grained, black to dark reddish in color, and fractures along 

 innumerable joint planes. The chert is usually rich in iron, and 

 sometimes contains enough lime to render the rock softer than 

 typical chert. Numerous small quartz veins cut the chert, 

 usually occupying joint cracks. An impure dark greenish 

 calcareous shale, approaching chert, occurs in the same locality 

 as the rock just described. It is seamed with calcite veins 

 carrying chalco pyrite, which stains the adjacent calcite green. 



A light greenish drab impure limestone mottled with reddish 

 blotches occurs in layers 2 or 3 feet thick interbedded with 



