REPORT OF THE CHIEF ASTRONOMER 569 



SESSIONAL PAPER No. 25a 



The new relation between the two Cordilleran belts was so similar to that 

 which obtained on the line of the Fortieth Parallel at the close of the Upper 

 Carboniferous period, that it is instructive to review King's statement, pub- 

 lished on pages 536-7 of the volume on Systematic Geology, Fortieth Parallel 

 Survey : — 



' After the close of this great conformable Palaeozoic deposition, wide- 

 spread mechanical disturbance occurred, by which the land-area west of 

 the Nevada Palaeozoic shore became depressed, while all the thickest part 

 of the Palaeozoic deposits from the Nevada shore eastward to and including 

 the Wahsatch, rose above the ocean and became a land-area. Between the 

 new continent and the old one which went down to the west, there was a 

 complete change of condition. The land became ocean; the ocean became 

 land. . . . 



1 Upon the western side of the new land-mass, the Archaean continent, 

 having gone down, made a new ocean-bottom, and upon this immediately 

 began to accumulate all the disintegration-products of the new land-mass 

 which the westward draining rivers and the ocean waves were able to 

 deliver. Throughout the Triassic and Jurassic periods the western ocean 

 was accumulating its enormously thick group of conformable sediments 

 upon the Archaean floor, . . until, at the close of the Jurassic age, there 

 had accumulated in the western sea 20,000 feet . . of Triassic and 

 Jurassic material.' 



6. During the Pennsylvanian period the Main Pacific Geosyncline was 

 the scene of heavy sedimentation with accompanying powerful vulcanism. 

 The rock exposures at the Forty-ninth Parallel do not suffice to show clearly 

 the dynamic events leading to the Triassic, but from Dawson's work in Van- 

 couver island as well as on the mainland, it appears that there was local defor- 

 mation of the Pennsylvanian beds in that part of the Cordillera, followed by 

 erosion of the upturned strata, before these were buried beneath Triassic 

 deposits. It is likely that the same crustal movement affected the Forty-ninth 

 Parallel section; and, further, that it is to be correlated with the beginning of 

 the Sierra Nevada downwarp described, as above, by King. How long or how 

 extensive was this temporary return to land conditions in the Western Belt 

 cannot be declared. It is known, however, that the Triassic period saw, at the 

 Forty-ninth Parallel, a resumption of marine sedimentation on the Pacific side 

 of the belt. Argillites, sandstones, and limestones, together with great piles 

 of basic volcanic material were then laid" upon the Pennsylvanian formations 

 in this region. 



7. The rocky record is blank for most of the Jurassic period, probably 

 indicating that an upheaval of the Triassic sea-bottom had begun, as an early 

 preparation for the late Jurassic revolution. This is the first general orogenic 

 revolution affecting the Western Cordilleran Belt since the Pre-Cambrian. It 

 was immediately followed by the intrusion of many large batholiths of grano- 

 diorite and allied rocks. Many of the larger batholiths between the Purcell 

 Trench and the open Pacific Ocean were intruded at this time. 



