Page:Walcott Cambrian Geology and Paleontology II.djvu/29

 Doctor Barren's view that the limestones are of marine origin is similar to the one that largely influenced me in the past to consider that the Grand Canyon and similar series of Algonkian formations were of marine origin.

In 1892 I published the opinion that the North American continent

Was larger at the beginning of Cambrian time than during any epoch of Paleozoic time and probably not until the development of the great fresh-water lakes of the lower Mesozoic was there such a broad expanse of land upon the continental platform between the Atlantic and Pacific oceans.

The continent was well outlined at the beginning of Cambrian time; and I strongly suspect, from the distribution of the Cambrian faunas upon the Atlantic coast, that ridges and barriers of the Algonkian continent rose above the sea, within the boundary of the continental plateau, that are now buried beneath the waters of the Atlantic. On the east and west of the continental area the pre-Cambrian land formed the mountain region, and over the interior a plateau existed that at the beginning of, or a little before. Upper Cambrian time was much as it is to-day. Subsequent mountain building has added to the bordering mountain ranges, but I doubt if the present ranges are as great as those of pre-Cambrian time that are now known only by more or less of their truncated bases. The Interior Continental area was outlined then and it has not changed materially since. Its foundations were built in Algonkian time on the Archean basement, and an immense period of continent growth and erosion elapsed before the first sand of Cambrian time was settled in its bed above them. [Walcott, 1892, pp. 562-563.]

When the Californian Cambrian sea began its invasion of the Algonkian land in southwestern California and Nevada there awaited the incoming waters a land surface deeply disintegrated and more or less base-leveled by erosion. As compared with the earlier epoch of the Algonkian it was a featureless continent, the elevations caused by folding and uplift in the geosynclines of the Cordilleran, Lake Superior, and Appalachian areas having been largely degraded. The rising waters of the Cambrian sea met with few marked elevations in the Cordilleran and Appalachian troughs, as is evidenced by the absence of coarse conglomerates and the presence above the basal conglomerates of immense deposits of very fine sandstone and mud rocks in the Lower Cambrian.

The character of the pre-Cambrian surface in the southern Appalachian area is well indicated in the description by Mr. Arthur Keith,