Page:Walcott Cambrian Geology and Paleontology II.djvu/25

 ], include in the Belt Mountain section 4,400 feet of limestone and 7,600 feet of arenaceous and siliceous strata [Walcott, 1899, p. 204], and in the Mission Range section about 6,000 feet of calcareous strata and over 18,000 feet of arenaceous and siliceous beds [Walcott, 1906, p. 18].

In the Lake Superior region the Keweenawan system is made up of a great series of mainly igneous rocks in the lower portion, with sedimentary sandstone and siliceous strata above, that form about 15,000 feet of the 50,000 feet in thickness provisionally assigned to this system [Chamberlin and Salisbury, 1906, p. 192].

The eastern or Atlantic Province Algonkian strata are represented in Newfoundland by about 10,000 feet of mainly siliceous sediments, and in Nova Scotia there may be 15,000 feet of similar sediments [Chamberlin and Salisbury, 1906, p. 204].

Sources of Sediments.—Of the sources of the sediments of the Algonkian (Proterozoic) formations of the Lake Superior region, Chamberlin and Salisbury consider [1906, Vol. 2, p. 199] "that a large portion of the sediments was produced by mature decomposition of older rocks, and this implies that they were not derived by rapid mechanical abrasion such as that which accompanies and follows great elevation and excessive precipitation. The great series of quartzites were derived from the complete decomposition of quartz-bearing rocks, and involved the almost complete separation of the quartz grains from other constituents, while the thick beds of shale arose from the complementary clayey products of decomposition from which most of the basic oxides had been removed by carbonation." These authors consider that the sediments were deposited on the bed of the Algonkian sea [p. 200].

The sources of the sediments of the Algonkian formations of the Cordilleran and Atlantic Provinces appear to have been similar to those described above, and all indicate relatively low elevations and quiet conditions of deposition except in the case of the massive, coarse sandstones in the lower Belt series of northwestern Montana [Walcott, 1906, p. 26].

The Algonkian formations of the Belt, Grand Canyon, Llano, and Avalon series all contain a large amount of bluish-black, red, and green, finely arenaceous, siliceous shales. These often form beds hundreds of feet in thickness, and extend over wide areas. With the exception of ''Cryptozoan ? occidentale'' Dawson [Walcott, 1906, p. 18] in some of the interbedded limestones there is no evidence of the presence of life in them.