REPORT OF TEE CEIEF ASTRONOMER 199 



SESSIONAL PAPER No. 25a 



the present line of contact between the Archean and the later terrane is not 

 itself the old shore-line. A careful study of the reports cited, has, however, 

 caused the writer to believe that, in general, the zone of shore-lines probably 

 lay not more than two or three score of miles to the southwestward of the 

 trench. 



From at least 60 D north latitude to about 52° north latitude the western 

 rim of the geosyncline ran roughly parallel to the course of the present Rocky 

 Mountain Trench. It is a question worthy of investigation whether there is 

 a genetic connection between the trench and this zone of shore-lines. Was the 

 line of dislocation established where it is because of the contrast of rigidity 

 between the strong rocks of the pre-Beltian and the weaker rocks of the geosyn- 

 cline? At the Forty-ninth Parallel the trench is at least 100 miles from the 

 zone of old shore-lines; it is possible that the specially thick and rigid Creston 

 and Kitchener quartzites functioned in the same way as the pre-Beltian rocks in 

 locating this main line of dislocation at the western edge of the weaker rocks 

 of the geosyncline, thus controlling the divergence of the shore-line zone and 

 the trench near the great bend of the Columbia river. 



Js"ot far north of the Sixtieth Parallel the Castle Mountain-Bow River group 

 of rocks disappears under newer formations and, as yet, the geosyncline cannot 

 be traced farther northwestward. 



Xowhere on the Canadian side has the eastern rim of the geosyncline been 

 discovered. The cover of Upper Paleozoic and Mesozoic formations will appar- 

 ently always forbid its discovery in Alberta and farther north. In the accom- 

 panying map (Figure 12) the eastern rim of the Belt-Cambrian portion of the 

 Rocky Mountain geosyncline is sketched in hypothetically. Its position is 

 marked almost wholly on the supposition that the width of the geosyncliue 

 remains fairly constant from the Forty-ninth Parallel northward. The notable 

 constancy in the lithological character and the great total thickness of the geosyn- 

 clinal beds where studied in the mountain belt from Montana to Yukon Terri- 

 tory, lends some colour to the supposition. That the geosynclinal holds its 

 width to 62° 30' north latitude is rendered almost certain by McConnell's dis- 

 covery of Castle Mountain dolomites and limestones on the Mackenzie river, 

 seventy miles below Fort Simpson.* 



In borings made by Baron von Hammerstein at the Athabaska river near 

 Fort McMurray, granitic and gneissic rocks, probably referable to the Archean, 

 were encountered at the depth of 1,000 feet, the overlying rock being Devonian 

 limestone. The Belt-Cambrian rocks seem thus, to be lacking at this point, 

 where their absence is possibly due to non-deposition. 



Southward from the International Boundary the geosyncline can be traced 

 with greater confidence. Like the Summit series of the southern Selkirks, the 

 Coeur d'Alene series, described by Messrs. Calkins, Ransome, and MacDonald, 

 and the equivalent Lolo series studied by Lindgren in sections farther south 



*E. G. McConnelL Ann. Report Geol. Snrv Canada, Vol. 4, 1888-9, Pt. D, p. 89 and 

 map. 



