REPORT OF THE CHIEF ASTRONOMER 203 



SESSIONAL PAPER No. 25a 



the Forty-ninth Parallel means, in the writer's view, that the pre-Tonto and 

 pre-Ignacio deformation affected the larger part of the southern end of the 

 Rocky Mountain Geosynclinal while the greater, northern part was not essenti- 

 ally affected by this phase of deformation. 



King's sections and context in the Fortieth Parallel survey reports clearly 

 show that the Belt-Cambrian geosynclinal was bounded on the east by land or 

 by marine shallows in the vicinity of the meridian of 110° west longitude at 

 the Uinta mountains.* This eastern rim of the geosynclinal seems to be the 

 western edge of an extensive land mass stretching from the Belt mountains 

 southward, as already described. 



In the latitude of the Uinta mountains the width of the Belt-Cambrian 

 geosyncline was about 375 miles; in the latitude of the Belt mountains it was 

 about 300 miles; its average width in the United States seems to have been 

 about 350 miles. In southern British Columbia and Alberta the width of tbe 

 exposed part of the geosynclinal is not more than 150 miles; at the Mackenzie 

 river the exposed part is about 225 miles wide. At both ends of the Canadian 

 portion of the geosynclinal and in all the stretch between, the actual width 

 was doubtless everywhere over 200 miles and, as noted above, is provisionally 

 assigned a magnitude similar to that observed in the United States. The 

 observed length of the geosyncline is 1,500 miles, and there are reasons for 

 believing that this huge sedimentary prism was yet longer, extending, at the 

 north, into Yukon Territory and at the south, into Arizona. The map, Fig. 12, 

 illustrates the fact, important to the theory of mountain-building, that the 

 axis of this old geosyncline ran faithfully parallel to the general axis of the 

 present Cordillera. 



The foregoing summary of many facts recently determined in Montana. 

 Idaho, and at the International Boundary, thus serves to confirm the view of 

 Dana, Dawson, King, and others concerning the existence of thick sedimentary 

 prisms, of which the Rocky Mountains of Canada and the United States, as 

 well as the ranges of the Great Basin, are largely composed. The present com- 

 pilation is intended principally to enforce the writer's belief that the Canadian 

 geosynclinal and the Fortieth Parallel geosynclinal are but parts of the same 

 thing. The great ( Belt terrane ' of Walcott is regarded by the writer as an 

 integral part of this immense sedimentary unit, being the stratigraphic equival- 

 ent of the Bow Biver-Castle Mountain series in the north, and of the con- 

 formable series below the Upper Cambrian in the Wasatch, Eureka, and other 

 districts of the Great Basin. 



Upper Paleozoic Portion of the Rocky Mountain Geosynclinal. 



We have seen that formations younger than the Middle Cambrian com- 

 pose but an insignificant fraction of the mountains crossed by the International 

 Boundary between the Great Plains and the summit of the Selkirk range. 



*See especially analvtical map and section facing p. 127 in King's Systematic 

 Geology, 1878. 



