12 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



was thus enlarged and deposition was generally continuous throughout the Cor- 

 dilleran belt until near the close of the Mississippian. Upon the Paleozoic beds 

 tnick Cretaceous and Tertiary prisms of sediment were locally laid down. Those 

 local geosynclinals and the master Rocky Mountain Greosynclinal compose the 

 Eastern Geosynclinal Belt of the Cordillera. 



Chapter IX. — Returning to the systematic description of the rocks, the 

 important Purcell Lava formation is here considered. Its characters in the 

 McGillivray, Galton, Clarke, and Lewis ranges are outlined. Certain associated 

 dikes and sills are described and the relation of this fissure-eruption to the thick 

 sills of the Yahk and Moyie ranges is discussed. 



Chapter X. — The intrusive gabbro sills of the Purcell mountain system 

 have already been described in preliminary papers. The matter of these pub- 

 lications, together with some new material, is presented in chapter X. It is 

 largely petrographic. A group of the most important intrusive bodies dis- 

 covered has been given the name, Moyie sills. It illustrates the ability of some 

 very thick magmatic sheets to assimilate their country-rocks — quartzites in the 

 case of the Moyie sills. The proofs of this are discussed in detail, and similar 

 cares are briefly compared. The principle of gravitative differentiation of 

 magmas is evident in all the cases. 



Chapter XL — The sedimentary rocks of the Nelson range, other than those 

 of the Summit series and some others intercalated in the Beaver Mountain 

 volcanic formation, include : the Kitchener quartzite, a small outlier of which 

 seems to be represented along the western edge of the Purcell Trench; the Priest 

 River terrane; and the Pend D'Oreille group. 



The pre-Cambrian Priest River terrane, the oldest rock-group identified 

 in the Boundary section, is composed of micaceous schists, quartzites, quartz 

 schists, dolomites, and metamorphosed greenstones, arranged in meridional 

 bands, but so complex in structure as to defy all attempts at deciphering their 

 true relation to one another. The petrography of the different bands is des- 

 cribed, and a note is added on the correlation of the terrane with others found 

 in the Cordillera north and south of the Boundary line. 



The Pend D'Oreille group is divided into the Pend D'Oreille limestone and 

 the Pend D'Oreille schist. As exposed in the Boundary belt, these rocks occur 

 in the batholithic province of West Kootenay, a fact which helps to explain the 

 heavy metamorphism of this group. The limestone is locally unfossiliferous 

 and, with some doubt, is correlated with the similar marbles of definitely Car- 

 boniferous age at Rossland. The schistose division includes phyllite, sheared 

 quartzite, amphibolite, and massive greenstone, which are intimately associated 

 with the limestone and are therefore tentatively referred to the upper Paleozoic. 



Then follows a brief analysis of the structure of the Nelson range (Selkirk 

 system). The Purcell Trench is located at the Forty-ninth Parallel on a fault 

 trough representing great vertical displacement. Horizontal shifts and a power- 

 ful overthrust, with rotation of the thrust-plane, are among the more important 

 structural elements in this area of strong deformation. 



