REPORT OF THE CHIEF ASTRONOMER 205 



SESSIONAL PAPER No. 25a 



lands to east and west during the Devonian or Mississippian periods, is not of 

 primary importance. The fact seems certain that the heaviest sedimentation of 

 those periods took place in the axial region of the ancient Cambrian down- 

 warp. The late Paleozoic (pre-Pennsylvanian) deposition, irregular as it may 

 have been, thus tended to complete the one massive prism out of which the 

 Rocky Mountains and the Great Basin ranges were later to be formed. The 

 southern part of the geosynclinal, that sectioned at the Fortieth Parallel of 

 latitude, for example, shows that the down-warping persisted into Pennsyl- 

 vanian time, but for the most part the Eastern Geosynclinal Belt of the Cordil- 

 lera seems to have been out of water during the Pennsylvanian. 



The records of the east and west transgressions of the sea during the 

 Devonian and Carboniferous periods tend, therefore, in a measure to obscure 

 the real situation of the sedimentary prism which was the essential antecedent 

 to the building of the Rocky Mountains of Alaska, Canada, and Montana, as 

 well as the ranges of the Great Basin. The western limit of the pre-Devonian 

 members of that prism is approximately the zone of shore-lines which has been 

 traced from southern California to the Yukon boundary. The zone may be 

 considered as including the rather indefinite line or limit separating the Eastern 

 Geosynclinal Belt of the Cordillera from the Western Geosynclinal Belt. That 

 line was, of course, neither straight nor smoothly curved. Bays of the Cam- 

 brian sea must have reached well into the western land on the west and we 

 have already seen that that land was extensively transgressed in the time 

 when the Rocky Mountain Geosynclinal, the essential sedimentary member of 

 the Eastern Belt, was being completed.* On the other hand, when the condi- 

 tions were reversed and the Eastern Belt, after upheaval, furnished detritus 

 out of which the geosynclinals of the Western Belt were constructed there were 

 deep bays running eastward into the land, and on the Fortieth Parallel, the 

 Eastern Belt was entirely covered by the sea. In spite of all these complica- 

 tions the division of the Cordillera into the two great belts tends to aid one in 

 the attempt to understand the true history of the Cordillera north of the 

 Mexican boundary. The division is made at the behest of dynamic geology, 

 not at that of paleogeography nor paleontology; in those groups of studies the 

 suggested division and nomenclature would probably have little value and 

 might even lead to confusion. In a word, the division is warranted only for 

 the geologist who is bent on locating geosynclinals, not shore-lines. The full 

 conception of the profound contrasts otherwise existing between the two belts 

 is not possible until a review is made of the diastrophic, igneous, and erosional 

 history of the Western Belt. 



* The discovery of Silurian and Devonian sediments in the Taylorsville district of 

 California, and again at one point in soiithwestern Alaska suggests that the Early 

 Paleozoic land mass of the Western Cordilleran Belt may have been locally interrupted 

 by straits connecting the marine area of the Rocky Mountain Geosyncline with the open 

 Pacific. 



