Page:Walcott Cambrian Geology and Paleontology II.djvu/34

 the west, the sediments of which are buried beneath later strata or are off the present shore line of the continent beneath the sea.

The theory that life originated and developed in fresh-water ponds and lakes does not appeal to me. More uniform conditions of temperature and environment would be present in the ocean and the sediments of the fresh-water deposits of pre-Cambrian and Cambrian time, if such exist, do not show sufficient evidence of life having existed at the time of their deposition. The Algonkian fossils of the Belt and Grand Canyon series [Walcott, 1899, pp. 227-239] probably came from the marine fauna when a temporary connection existed between the interior fresh- or brackish-water lakes and the ocean.

1. Life probably first developed in the open ocean, as outlined by Brooks [1894].

2. The life of the oceans became adapted to littoral and shore conditions in Algonkian time during a period when the relation of all the continents to sea level was essentially the same as at the present time, or the continents may have been still more elevated in relation to the surrounding oceans.

3. The period of Algonkian continental elevation was of sufficient duration to permit of the development along the shores and shelves of the continents of the types of life now found in the basal Cambrian rocks, but the sediments containing the record of this life are probably concealed beneath the present oceans.

4. The known fossils contained in the Algonkian sediments of the Cordilleran geosyncline lived in fresh or brackish waters that were rarely in connection with marine waters on the margins of the Algonkian continent of North America. This will explain the abrupt appearance of a highly specialized crustacean deep down in the Belt series.

5. When the oceanic waters gained access to the Algonkian continental areas at the close of that era they brought with them the littoral fauna which had been developed during the Lipalian sedimentation, and buried its remains in the sands and muds which form the Lower Cambrian deposits.