12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5/ 



British Columbia and Alberta, on the main line of the Canadian 

 Pacific Railway, to Arizona and southern California, a distance of 

 over I, GOO miles, I have found evidence of a transgressing Cambrian 

 sea and consequent unconformity between the Cambrian and pre- 

 Cambrian. It may have been the advancing, overlapping Lower 

 Cambrian sea as in southwestern Nevada, the Middle Cambrian sea 

 as in Utah and Idaho, or the Upper Cambrian sea as in Colorado. 



The Cambrian rocks may be abruptly unconformable upon the 

 Algonkian [Walcott, 1891, p. 551, fig. 48], or practically conformable 

 as in areas where there has been very little disturbance of the sub- 

 jacent Algonkian beds [Walcott, 1899, pp. 210-213]. Over the 

 interior of the continent the late Middle Cambrian and Upper Cam- 

 brian strata unconformably overlap the Algonkian and Archean 

 [Walcott, 1891, pi. 44, pp. 561-562], and clearly could not have re- 

 corded any part of the history of the period indicated by the absence 

 of Lower Cambrian strata or of the sediments deposited in the 

 period represented by the unconformity between the Lower Cambrian 

 and Algonkian strata. I do not know of a case of proven conformity 

 between Cambrian and pre-Cambrian Algonkian rocks on the North 

 American continent. In all localities where the contact is sufficiently 

 extensive, or where fossils have been found in the basal Cambrian 

 beds or above the basal conglomerate and coarser sandstones, an 

 unconformity has been found to exist. Stated in another way, the 

 pre-Cambrian land surface was formed of sedimentary, eruptive, and 

 crystalline rocks that did not in any known instance immediately 

 precede in deposition or origin the Cambrian sediments. Everywhere 

 there is a stratigraphic and time break between the known pre- 

 Cambrian rocks and Cambrian sediments of the North American 

 continent. 



EXTENT OF WITHDRAWAL OF SEAS IN ALGONKIAN 



TIME 



That the present area of the North American continent was higher 

 than the level of the Atlantic and Pacific oceans at the beginning of 

 known Cambrian time is, I think, well established, and with the data 

 available it would appear that all other continental areas were in a 

 similar condition. What diastrophic action caused the withdrawal of 

 the oceanic waters from the continental areas during the great pe- 

 riod represented by the non-marine deposition of the later Algonkian 

 sediments and the period of erosion preceding the deposition of the 

 superjacent Cambrian sediments, is unknown. It may have been pro- 



