568 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



land may have had a width approximating that of the Cordilleran division here 

 recognized as the Western Geosynclinal Belt. 



The Rocky Mountain Geosyneline was doubtless limited, also on the east, 

 by definite shore-lines like those proved to have existed in the Belt mountains 

 not far south of the International Boundary. 



On both sides of the geosyneline there were specially important trans- 

 gressions of the sea during the Middle Cambrian (Flathead), and the Mississip- 

 pian periods. At those times the downwarped area was notably widened but 

 in neither case did it lose its character of a geosyneline with north-northwest 

 and south-southeast axis. 



In the geosynclinal area contemporary vulganism interrupted the deposition 

 of clastic or chemical sediments at several different periods. The first volcanic 

 activity broke out soon after the initiation of the downwarp, developing the Irene 

 Volcanic formation. Less important eruptions occurred during the deposition 

 of the Grinnell, Sheppard, and Kintla formations, while the singularly persistent 

 Purcell Lava was poured out at the close of the Siyeh stage. It is probable 

 that this last lava is contemporaneous with the thick sills of gabbro which 

 have been thrust into the bedding-planes of the quartzites in the Purcell moun- 

 tain system. 



4. Up to this time we have no record except that of steady erosion for the 

 Western Cordilleran Belt east of the Gulf of Georgia. The great volume of 

 the Rocky Mountain Geosynclinal implies that the western land-area was steadily 

 or spasmodically rising during this deposition of clastic material on the east. 

 It is quite possible that during the Mississippian period, the already heavily 

 eroded land of the Western Belt was temporarily submerged and received a 

 marine sedimentary veneer; but of this we have no paleontological evidence 

 on or near the Forty -ninth Parallel. 



It is also possible that some of the oldest greenstones of the Columbia 

 mountain system and of the Interior Plateaus region are of pre-Pennsylvanian 

 dates, representing vulcanism on the land-surface of the Western Belt. 



5. At or near the close of the Mississippian period the Western Cordilleran 

 Belt was certainly submerged, and the Eastern Geosynclinal Belt was broadly 

 upwarped, without other general deformation of the Rocky Mountain Geo- 

 synclinal. The Main Pacific Geosyneline was thus initiated or else deepened, 

 so as to receive a great load of Pennsylvanian sediments. Fossiliferous beds 

 belonging to this period have been found at intervals in the Western Belt from the 

 Columbia river to Vancouver island. So far as they are clastic their materials 

 seem to have been derived from the newly emerged Eastern Belt. The ancient 

 relation of the two belts was thus reversed, except for local, temporary embay- 

 ments on the east. This movement was, apparently, felt from Alaska to northern 

 Utah at least; farther south, in the region of the Fortieth Parallel, the reversal 

 of relations was postponed to the close of the Pennsylvanian period. Other- 

 wise the Eastern and Western Belts have respectively behaved as units in the 

 momentous change. The larger part of the Eastern Belt was to remain as land 

 through the Permian, Triassic, and most of the Jurassic periods; and even in 

 the later periods to undergo only partial submergence. 



