450 STUDIES FOR STUDENTS 



or no more metamorphosed than itself, as in Montana and in Arizona. 

 More frequently, however, over all the continents it rests directly 

 upon steeply dipping gneisses and schists. These structures, char- 

 acteristic of the zone of rock-flowage, indicate widespread and pro- 

 found erosion previous to the Cambrian transgression, and there- 

 fore a period of wide and long-enduring continental extension. 



The usual concept of the development of the North American 

 continent embodied in the textbooks of the past begins with the 

 widespread submergence of the late Cambrian and early Ordovician. 

 From that stage Dana has long since shown how the continent 

 through the Paleozoic gradually, and with many regressions, gained 

 in dry land. 



Walcott considers, however, that the prevailing view of the geo- 

 graphic distribution and extent of the continental area at the begin- 

 ning of Paleozoic time is too restricted, and that the continent was 

 larger at the beginning of the Cambrian period than during any 

 subsequent epoch of Paleozoic time.^ At this date (1891) Walcott 

 had not yet separated the underlying belt formation of Montana 

 from the Cambrian, and consequently considers this portion of the 

 Northwest to have -been beneath the sea during Lower Cambrian 

 time. With the discovery of an uncomformity separating the Belt 

 from the Middle Cambrian,^ this statement in regard to the extent 

 of the early Cambrian land is further justified, and will probably be 

 considered by those familiar with the subject as very conservative. 



LeConte also emphasizes the significance of the Pre-Cambrian 

 record of erosion, the subject being briefly stated in his Elements 0} 

 Geology. 



A truer appreciation of the facts of the world-wide Pre-Cambrian 

 unconformity may finally lead to the erection of this Pre-Cambrian 

 period into an aeon of continental extension as widespread and as 

 long-enduring as that of the Mesozoic and Cenozoic. 



Yet it is only the end of this period, as marked by the Lower 

 Cambrian unconformity, which is known with any accuracy. On 

 going backward in time, evidences of repeated orogenic movements 



1 "The North American Continent during Cambrian Time," Twelfth Annual 

 Report (1891), U. S. Geological Survey, Part I, p. 562. 



2 C. D. Walcott, "Pre-Cambrian Fossiliferous Formations," Bulletin of the Geo- 

 logical Society of America, Vol. X (1898), p. 210. 



