REPORT OF TEE CEIEF ASTRONOMER 565 



SESSIONAL PAPER No. 25a 



General Features op the Western Geosynclinal Belt. 



The various tables illustrate fairly well the safer generalizations which 

 may now be made concerning the geological history of the Western Geosynclinal 

 Belt. 



1. The western one-third or two-fifths of the Cordillera forms a belt of great 

 rock-formations which together constitute a single geological province. These 

 formations are uniform in their very diversity as they are followed from south- 

 eastern Alaska, or indeed, from northwestern Alaska, to middle and southern 

 California. 



2. It is equally clear that the Western Belt is in deep contrast with the 

 Eastern Belt and that in a large way the two are in reciprocal relations. 

 The area covered by the Western Belt has furnished most of the clastic material 

 in the principal geosynclinal of the Eastern Belt; the Eastern Belt has furnished 

 most of the clastic material composing the principal geosynclinal of the Western 

 Belt. 



3. The dominant sediments in the Western Belt seem undoubtedly to be 

 the Upper Carboniferous, the Triassic, and, in less degree, the Jurassic. The 

 Pennsylvanian beds seem to be absolutely continuous from southern California 

 to Alaska and to have unusual thicknesses in all the more complete sections 

 known. They and the locally underlying Mississippian as well as older beds 

 compose the broad, fundamental prism of sediments out of which the post- 

 Jurassic mountains were made. This whole older group of beds may be called 

 for convenience, the Carboniferous Geosynclinal. 



The generally conformable Jurassic-Triassic series, totalling great thick- 

 nesses, especially in California, has been proved at a few points to be uncon- 

 formable upon the Pennsylvanian but the unconformity may not be very great. 

 The rocks of all three systems are most intricately involved with one another 

 and have shared in the paroxysmal movements of Jurassic and later time, as 

 if they had all belonged to one conformable group. While, therefore, we may 

 refer to the Jura-Trias beds under the name ' Jura-Trias Geosynclinal,' it will 

 be convenient to have a name for the entire series of Carboniferous (and older?), 

 Triassic, and Jurassic strata which have co-operated in the making of the larger 

 sedimentary complex of the Western Belt of the Cordillera. The complex may 

 be called the ' Main Pacific Geosynclinal.' 



The enormously thick prisms of clastic deposits laid down in the Cretaceous 

 are distinctly local and are separated by first-class unconformities from Jura- 

 Trias-Carboniferous on the one hand, and Tertiary formations on the other. 

 All of these prisms may be called Cretaceous geosynclinals and each may be 

 given a geographical name, e.g., Pasayten geosynclinal, Shasta geosynclinal, 

 Qneen Charlotte geosynclinal, etc. 



Similarly, the heavy Eocene deposits may be called the Puget geosynclinal, 

 Arago geosynclinal, etc. 



The stratified deposits of the other periods are not recorded to have suffi- 

 cient thickness to warrant our recognizing true geosynclinals, i.e., bodies thick 



25a— vol. iii— 37J 



