Page:Walcott Cambrian Geology and Paleontology II.djvu/28

 I find in my field notes of 1879, on the Tertiary section at the head of the Upper Kanab Valley in southern Utah, the following on the fresh-water beds:

The limestones extend over a considerable area west, north, and east, and were deposited in massive layers in a deep, quiet lake.

The Tertiary sedimentation described above, omitting the eruptive materials, is very similar in many respects to that of later Algonkian time. The sediments of the Algonkian are as a whole more siliceous, but the variation in thickness and character of the various beds [Walcott, 1906, pp. 17-21] is of the same general type.

The sediments of the two widely separated periods of Algonkian and Tertiary time were accumulated within the limits of the great Cordilleran geosyncline, and, with our present knowledge, I think, under essentially similar physical conditions. At the time of the Tertiary deposition there was abundant life, both on the land and in the water, but in Algonkian time only a fragment of the pre-Cambrian life had had the opportunity of adapting itself to the conditions of the inland seas of late Algonkian time.

Dr. Joseph Barrell has given a full review of the evidence favoring the continental origin of most of the Algonkian rocks of the Cordilleran area. He argues that from the presence of mud-cracks in the Belt and Grand Canyon series that many of the formations were deposited on flood plains. He says [1906, p. 566]:

The discussion of these pre-Cambrian deposits but especially of the Montana occurrences, shows how completely in accord is the hypothesis of the dominant flood-plain origin of mud-cracks with the other marks of subaerial deposition in an arid climate. The mud-cracks are confined to just such formations as from other characteristics suggest a flood-plain origin and these formations are usually separated from the deposits of limestone by transitional formations which differ in color, in character, and in the absence of mud-cracks, suggesting the true submarine deposits originating between the shore and the open sea.