REPORT OF THE CHIEF ASTRONOMER 195 



SESSIONAL PAPER No. 25a 



Eastern Geosynclinal Belt of the Cordillera. 



In the final generalization regarding a mountain-chain, namely, the theory 

 of its origin, it is of first importance to include a definite conception of the 

 geosynclinal sedimentary prism or prisms out of which the rock folds of that 

 chain have been made. For the eastern half of the North American Cordillera 

 the complex orogenic history must he discussed in terms of at least three periods 

 of specially important geosynclinal sedimentation. As Dana long ago pointed 

 out, the principal period is that of the deposition of the stratified series from the 

 Cambrian (and conformable pre-Olenellus) system to the Mississippian system 

 inclusive. For this huge accumulation of clastic and chemical deposits the 

 present writer has proposed the name ' Rocky Mountain Geosynclinal ' ; the 

 down-warped surface of the pre-Cambrian on which the prism rests may be 

 said, for distinction, to form the ' Rocky Mountain Geosyncline.' 



In northern Alaska, northeastern Alaska, eastern Yukon, eastern British 

 Columbia, Alberta, Montana and central Utah, the Rocky Mountains, in the 

 common and narrower sense of the term, are chiefly or largely composed of rocks 

 forming part of this prism. So far the proposed name is appropriate. In Colorado 

 the Rocky Mountains are principally composed of other terranes, so that the 

 folded and faulted rocks of the prism constitute the ranges of the Great Basin, 

 all of which lie well back of the front range of the Rockies proper. For this 

 part of the Cordillera the proposed name is not fitting, except as the prism 

 is, by its name, located alongside the local range of the true Rocky Mountains. 

 However, the fact that by far the greatest part of the Rocky Mountain chain 

 fproper) is actually made of the rocks of this prism, has impelled the writer 

 to suggest the name chosen. Dana has offered the name ' Rocky Mountain 

 geosynclines ' for the post-Cretaceous down-warps affecting a local part of the 

 Cordillera, namely, that in the Wasatch-Green river region.* For the student 

 of continental geology this name seems hardly appropriate; the larger part of 

 the Rocky Mountain group has not been affected by down-warps of this date, 

 at least to the extent demanding the formation of thick prisms of sediment. 

 In any case the main uplift of the Rockies proper has not been due to the 

 generation of Tertiary geosynclinals but has rather been one of the causes of 

 their subsequent formation. 



During the other two periods of heavy sedimentation, the resulting geosyn- 

 clinals were incomparably smaller and all of more local nature than the 

 enormous mass of strata upon the back of which, and from the substance of 

 which, these younger prisms were made. The latter include the Cretaceous 

 geosynclinal of the Crowsnest district in Canada as well as that in Colorado; 

 also the Eocene geosynclinals of the great down-warps north and south of the 

 Uinta mountains. 



With the Cretaceous and Eocene geosynclinals we are not now engaged, 

 but they are mentioned in this place in order to indicate once more the advis- 

 ability of having a convenient name for the eastern half of the Cordillera which 



*J. D._Dana, Manual of Geology, 4th edition, 1895, p. 365. 

 25a — vol. ii — 13A 



