Page:Walcott Cambrian Geology and Paleontology II.djvu/32

 British Columbia and Alberta, on the main line of the Canadian Pacific Railway, to Arizona and southern California, a distance of over 1,000 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 subjacent Algonkian beds [Walcott, 1899, pp. 210-213]. Over the interior of the continent the late Middle Cambrian and Upper Cambrian strata unconformably overlap the Algonkian and Archean [Walcott, 1891, pi. 44, pp. 561-562], and clearly could not have recorded 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.

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 period 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