Page:Walcott Cambrian Geology and Paleontology II.djvu/20

 In a word, the thought is that the Algonkian period, with its great epicontinental formations, was a period of continental elevation and largely terrigenous sedimentation in non-marine bodies of water, also a period of deposition by aerial and stream processes over considerable areas.

The recent map of the pre-Cambrian rocks of North America by Van Hise and Leith [1909, pl. 1] shows that the distribution of the Algonkian rocks is confined to areas well within the margins of the continental platform. The strata we have to consider on this map are in the areas placed under the Algonkian, including Keewatin areas in Canada and the Lake Superior region. Van Hise and Leith [1909, p. 21] define the Algonkian as including "the major part of the pre-Cambrian sedimentary rocks, though it also contains sediments so deformed and metamorphosed that their stratigraphy cannot be deciphered. The Archean is the basement complex, perhaps including several series or groups upon which the Algonkian rests, so far as known, with unconformity."

The sediments of other Algonkian areas of North America were much like those of the Lake Superior region in having immense quantities of sand and mud, and, more rarely, great thicknesses of calcareous matter.

Briefiy, the rocks of the later Algonkian formations may be outlined as follows:

There are no conglomerates except a bed not exceeding 30 feet in thickness at the base.

The Llano series of Texas is much like the Grand Canyon series [Walcott, 1884, pp. 431, 432].

The Belt series of Montana [Walcott, 1899, pp. 201-209; 1906, pp. 2-15, 17-21] and British Columbia, Canada [Walcott, 1906, pp.