EVIDENCES OF PRIMITIVE LIFE WALCOTT. 237 



The North American continent was larger at the beginning of 

 known Cambrian time than at any subsequent period, other than 

 possibly at the end of Paleozoic time and the end of Cretaceous time, 

 when the land area was equally extensive. Indeed, it is highly prob- 

 able that its area- was even greater then than now, for no marine 

 deposits containing pre-Cambrian life, as they were laid down in 

 Lipalian^ time immediately preceding the Cambrian period, have 

 been discovered in the North American Continent or elsewhere, so 

 far as known. 



I gradually came to the conclusion- that the most natural expla- 

 nation of the absence of the traces of a distinct marine pre-Cambrian 

 fauna is that the North American continent in pre-Cambrian time 

 was at such an elevation above the sea that there is now no record 

 of the sediments deposited on the under sea shelf about the conti- 

 nental area of that time. This presupposes that the great series of 

 pre-Cambrian Algonkian sediments in the Rocky Mountain region 

 were deposited in an inland mediterranean, or a series of great lakes 

 and flood plains such as existed in Tertiary times.^ The same con- 

 clusion applies to all of the later pre-Cambrian Algonkian forma- 

 tions of the Lake Superior region, Texas, Arizona, and so forth. 



On this hypothesis the evolution of the pre-Cambrian fauna was 

 taking place in waters contiguous to the continental area, and their 

 remains, buried in the sediments then accumulating, have not been 

 found, owing to the fact that those sediments are now hidden beneath 

 the sea off the present coast lines of the continent. That such a con- 

 dition existed is suggested by the almost total absence of any traces 

 of life in the existing pre-Cambrian sediments. 



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 with- 



1 Abrupt appearance of the Cambrian fauna on the North American Continent. Smith- 

 sonian Misc. Coll., vol. 57, no. 1, 1910, p. 14 (footnote). Lipalian (XeiTro+aXs) was pro- 

 posed for the era of imlinown sedimentation between the adjustment of pelagic life to 

 littoral conditions and the appeai'ance of the Lower Cambrian fauna. It represents the 

 period between the formation of the Algonkian continents and the earliest encroachment 

 of the Lower Cambrian sea. 



- Olenellus and other genera of the Mesonacida?, Smithsonian Misc. Coll., vol. 53, no. 6, 

 1910. 



*The crustacean and annelid faunas described from these sediments [Walcott, 1899, 

 Pre-Cambrian fossiliferous formations, Bull. Geol. Soc. America, vol, 10, p. 238] might 

 quite as well have been fresh-water as marine forms. There is nothing as far as known 

 to indicate that they were necessarily limited to a marine habitat. 



* Abrupt appearance of the Cambrian fauna on the North American Continent. Smith- 

 sonian Misc. Coll., vol. 57, no, 1, 1910, p. 12. 



