2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. gi 



tana. North of the Belt Mountains or of the latitude passing through 

 Helena, Mont., no Middle Cambrian was known in the vast expanse 

 of Beltian sediments extending far north into Canada, except that 

 discovered in 1905 by Walcott in a limited area near Ovando, Lewis 

 and Clark County, north of Helena. In the Canadian Rockies Walcott 

 traced similar Middle Cambrian beds southward at least as far as 

 Elk Pass on the Continental Divide. Thus it is evident that the gap 

 between the nearest exposures of Middle Cambrian in the Rockies of 

 the United States and of Canada was much smaller — by the distance 

 between Salt Lake City and the Ovando area — than that between the 

 nearest exposures of the Lower Cambrian. 



Finally, the Upper Cambrian was known to extend rather generally 

 throughout the southern Rocky Mountains, as defined below, where 

 it constitutes the sole Cambrian deposition. These strata are at present 

 best designated as the Deadwood series. In Montana the upper portion 

 of Peale's Gallatin limestone series is of about the same age as the 

 Cambrian in the Southern Rockies, and in Canada beds corresponding 

 rather closely to the Gallatin series and younger strata are well de- 

 veloped. However, both in Montana and in Canada the Upper Cam- 

 brian has a more restricted distribution than the Middle Cambrian 

 and does not exactly coincide with it. Thus, earlier observations 

 indicated that Cambrian outcrops were confined to the Rocky Mountain 

 system proper — as defined in the following paragraphs — and that in 

 it an extensive area existed in western Montana, northern Idaho, and 

 the southern parts of Alberta and British Columbia, in which Cambrian 

 strata were apparently lacking. 



PRIMARY STRUCTURAL UNITS OF THE ROCKY MOUNTAINS 



Before the recent discoveries are described, a few words concern- 

 ing the fundamental structure of this part of the Cordilleran region 

 will be helpful. In the light of early Paleozoic history it is desirable 

 to depart somewhat from the regional classification in vogue, which 

 is based primarily on present topography and is, therefore, a delimi- 

 tation of physiographic rather than of structural provinces. The 

 structural provinces, as here outlined, take account of the persistently 

 positive and negative elements, and amounts of total and dififerential 

 movement, or, in short, the geographic conditions during the initiation 

 of the Cambrian or of other initial early Paleozoic periods, in so far 

 as they are determinable. 



Southern Rocky Mouutains. — According to conditions at the begin- 

 ning of Paleozoic time, which persisted throughout that era, this 



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