532 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



before the peridotite was injected. The proximity of the Skagit volcanics over- 

 lying the gneiss leads one to suspect that these basic rocks belong to the same 

 eruptive period, and that the harzburgite and andesites are genetically connected. 

 The relation is conceivably the same as that connecting the peridotites of the 

 Columbia range and Midway mountains with the basalts and andesites of those 

 regions. 



Slesse Diorite. 



The walls of Middle creek canyon in its lower part are composed of diorite, 

 which extends over the divide past Slesse mountain to Slesse creek. The diorite 

 forms a stock-like mass covering about nine square miles on the Canadian side 

 of the line; it was not mapped to the southward, but it is known to extend 

 several miles into Washington. The diorite body once undoubtedly stretched 

 farther eastward, but it has there been replaced by the younger Chilliwack batho- 

 lith of granodiorite. 



The diorite is very clearly intrusive into the slates on the west and north. 

 These argillites are highly altered, but, as they enclose lenses of crinoidal Car- 

 boniferous limestone, it seems most probable that the date of intrusion is post- 

 Carboniferous. 



The diorite is not crushed or greatly strained except in the immediate 

 vicinity of the great Chilliwack batholith, where such effects might naturally be 

 expected. Elsewhere there are no evidences that the diorite has undergone the 

 severe pressures involved in the post-Cretaceous mountain-building of the 

 Cascade range; it is therefore probable, though not proved, that the diorite was 

 intruded in post-Laramie time. 



The contacts of the body are so imperfectly exposed in this densely forested 

 area that its structural relations have not been fully worked out. The diorite 

 certainly cross-cuts the sedimentaries and has metamorphosed them in the 

 thorough way characteristic of most stocks. The intensity of the metamorphism 

 is of a higher order than that usually observed about a laccolith or chonolith, 

 and it seems safer to regard the mass as a true stock or batholith, that is, a sub- 

 jacent, downwardly enlarging body. 



The diorite is in places richly charged with large, slab-like inclusions of 

 crumpled black slate; these often attain lengths of 50 to 100 feet or more. A 

 large number of them, forming a veritable breccia on a great scale, may be seen 

 on both slopes of Middle creek canyon, especially at points about four miles 

 from the confluence of the creek with the Chilliwack river. 



Petrography. — The diorite is a dark brownish to greenish gray, fresh rock 

 of normal habit. It appears to have a rather uniform chemical composition. 

 The chief variations are those of grain. At its own intrusive contacts the stock 

 is fine-grained as if by chilling; elsewhere the grain is generally of medium 

 size. Where the diorite contacts with the younger granodiorite the grain is still 

 medium, but the more basic rock has been metamorphosed along a narrow zone. 

 Basic segregations were not observed in the diorite. 



The list of essential minerals in the diorite includes acid labradorite, near 

 Ab 4 An 3 , hornblende, and biotite, named in the order of decreasing abundance. 



