Evidences of primitive life / by Charles D. Walcott.

  • Charles Doolittle Walcott
Date:
1916
Catalogue details

Licence: In copyright

Credit: Evidences of primitive life / by Charles D. Walcott. Source: Wellcome Collection.

    13/58 page 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 Lipalian1 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 conclusion2 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.3 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.4 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 (Xei7ra+aXs) was pro¬ posed for the era of unknown sedimentation between the adjustment of pelagic life to littoral conditions and the appearance 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. 2 Olenellus and other genera of the Mesonacidse, Smithsonian Misc. Coll., vol. 53, no. 6, 1910. 3 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. 4 Abrupt appearance of the Cambrian fauna on the North American Continent. Smith¬ sonian Misc. Coll., vol. 57, no. 1, 1910, p. 12.
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