REPORT OF THE CHIEF ASTRONOMER 1 1 



SESSIONAL PAPER No. 25a 



Purcell system is the presence of thick sills of a peculiar hornblende gabbro. 

 These eruptive formations are described in later chapters. 



The Purcell system is also characterized by numerous examples of block- 

 faulting, though the McGillivray range shows a broad anticline and syncline. 



Chapter VII. — At the Purcell Trench the continuity of the geosynclinal 

 mass is effectively broken. Prom the alluviated floor of the trench to a line 

 about sixteen miles farther west the rocks chiefly belong to the older Priest 

 River terrane, on which the geosynclinal was deposited. At the summit of the 

 Nelson rauge the nearly entire thickness of the geosynclinal is exposed, the 

 prism having here been upturned to a vertical position. Its sedimentary mem- 

 bers are heterogeneous, including conglomerates, grits, coarse and fine sand- 

 stone (quartzites), and metargillites. A very thick volcanic formation, older 

 than the Purcell Lava, is interbedded. A great unconformity at the base of the 

 geosynclinal is exposed. The name Summit series is given to the whole con- 

 formable group of formations, from the basal unconformity to the horizon 

 corresponding to the youngest member of the Purcell series. West of the great 

 monocline the Summit series makes an apparently conformable contact with 

 a younger metamorphosed mass of sediments named the Pend D'Oreille group. 

 West of that contact the Rocky Mountain Geosynclinal rocks do not reappear 

 in the Boundary section. 



Chapter VIII. — In this chapter the detailed description of the Selkirk 

 geology is interrupted, and the correlation of the Lewis, Galton, Purcell, and 

 Summit series is discussed. The systematic variation in the lithology of the 

 geosynclinal, as it is crossed from east to west, is noted in some detail, and the 

 conclusion is drawn that the source of the clastic materials lay to the westward, 

 probably not far from the present location of the Columbia river. Notes on the 

 metamorphi-m of the prism and on its average specific gravity are entered. 



The lithological correlation of the geosynclinal with the Cambrian forma- 

 tions described by McConnell and Walcott on the Canadian Pacific railway is 

 then discussed. The result is to point to the probability that the geosynclinal 

 at the International Boundary is largely Cambrian in age, though its basal 

 members belong to pre-Olenellus horizons (Beltian of Walcott). Similar 

 correlation with sections described in Montana and Idaho suggests a similar 

 conclusion as to the age of the sediments in the four Boundary series, and it 

 is held that a considerable thickness of the ' Belt terrane ' is possibly, if not 

 probably, of Middle and Lower Cambrian age. 



The chapter closes with an outline of the argument that the eastern half of 

 the Cordillera, from Alaska to Arizona and including the Great Basin of the 

 United States, has been the scene of specially heavy sedimentation during the 

 Beltian, Lower Cambrian, and Middle Cambrian periods. The lower part of the 

 Rocky Mountain Geosynclinal, as defined, has an axial trend faithfully parallel 

 to the main Cordilleran axis of the present day. This geosynclinal suffered a 

 local deformation during an early Middle Cambrian period, and, at the Middle 

 Cambrian Platbead stage, was generally depressed. The area of sedimentation 



