282 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V, A. 1912 

 Metamorphosed Basic Intrusives in the Priest Kiver Terrane. 



Various belts of the Priest River sedimentaries enclose narrow dikes and 

 sills of basic igneous rocks and one or two basic bodies of larger size. With few 

 exceptions these are poorly exposed and the intrusives are enormously altered. 

 It is, therefore, impossible to give a satisfactory account of the intrusives either 

 as to the field relations of several of the bodies or as to the original nature of the 

 magmas whence they have been derived. 



The largest of the bodies outcrops for a distance of several hundred yards 

 on the trail running from Boundary lake to Summit creek and at a distance of 

 about 2,000 yards west of the top of North Star mountain. This body is at least 

 500 feet broad. Whether it is a great dike or sill or an irregular intrusion could 

 not be determined. The rock is a dark green, fine-grained, highly schistose trap. 

 Labradorite in small broken individuals is apparently the only primary mineral 

 remaining after the profound metamorphism that the rock has undergone. Most 

 of it is now a mass of chlorite, uralite, epidote, secondary quartz, leucoxene, and 

 pyrite. The original structure seems to have been the hypidiomorphic-granular. 

 The rock was doubtless a gabbro, now altered to a chlorite-uralite-labradorite 

 schist or greenstone. 



A ten-foot sill-like intrusion of a somewhat similar rock was found in belt 

 E where it crosses the main fork of Corn creek. 



Just below the lower contact of the Irene conglomerate on Summit creek, 

 belt A of the Priest River terrane is cut by a relatively uncrushed hornbiendite, 

 occurring as a sill three feet in thickness. The essential amphibole has nearly 

 the same optical properties as the hornblende of the Purcell sill gabbros. Feld- 

 spar is absent. Magnetite and apatite are the observed accessories. Chlorite, 

 quartz, and a little carbonate are secondary products. A larger sill-like body, at 

 least 100 feet thick, cuts the quartzites of belt D at the junction of the North 

 Fork and main fork of Summit creek. This rock bears much quartz, orthoclase, 

 £:nd some indeterminable plagioclase, along with the dominant green hornblende. 

 Its composition and habit recall the acidified hornblende gabbro of the Purcell 

 sills. 



A quarter of a mile from the Rykert granite contact the schists of belt F are 

 cut by a 125-foot sill of originally basic igneous rock which is now a dark green 

 amphibolite, composed essentially of green hornblende, quartz, and basic plagio- 

 clase (labradorite to bytownite) along with much accessory orthoclase and red 

 garnet. This sill has been squeezed to a highly schistose condition and thor- 

 oughly metamorphosed during the intrusion of the Rykert granite. 



Beyond the fact that these intrusives cut the Priest River sedimentaries, 

 there is little direct evidence as to their age. The thoroughness of their dynamic 

 metamorphism indicates a pre-Tertiary age, while the lithological similarity of 

 the gabbroid bodies to the gabbro of the Purcell sills suggests the possibility that 

 the former may also be as old as the Middle Cambrian, to which the Purcell sills 

 have been tentatively referred. Some of these intrusives may represent the deep- 

 seated phase of the yet older Irene volcanies. In any case the impression won 

 in the field was that the chlorite-uralite schist, the amphibolite, and the sheared 

 hornblende gabbro are of much older date than any other of the igneous bodies 



