Page:Walcott Cambrian Geology and Paleontology II.djvu/26

 There are also thick masses of limestone interbedded in the Algonkian series. The Blackfoot terrane has over 4,800 feet of calcareous beds. The Siyeh limestone is 4,000 feet thick, and the Newland limestone 2,000 feet. Similar blue, red, and green deposits, if deposited in the ocean, would probably have been accumulated in deep, quiet water [Chamberlin and Salisbury, Vol. 1, 1904, pp. 361-363], but (a) the large amount of calcareous matter; (b) the presence of shrinkage cracks and ripple markings on shales, sandstones, and limestone shales; (c) the presence in calcareous shales of fossils that lived in shallow water [Walcott, 1899, pp. 235-238], indicate that the Algonkian sediments were deposited in relatively shallow water.

In speaking of the climatic significance of red-colored deposits, Barrell [Barrell, 1908, pp. 292 and 293] says:

Turning to the climatic significance of red, it would therefore appear both from theoretical considerations and geological observations that the chief condition for the formation of red shales and sandstones is merely the alternation of seasons of warmth and dryness with seasons of flood, by means of which hydration, but especially oxidation of the ferruginous material in the flood-plain deposits is accomplished. This supplements the decomposition of the source and that which takes place in the long transportation and great wear to which the larger rivers subject the detritus rolled along their beds. The annual wetting, drying, and oxidation not only decompose the original iron minerals but completely remove all traces of carbon. If this conclusion be correct, red shales or sandstones, as distinct from red mud and sand, may originate under intermittently rainy, subarid, or arid climates without any close relation to temperature and typically as fluvial and pluvial deposits upon the land, though to a limited extent as fluviatile sediments coming to rest upon the bottom of the shallow sea. The origin of such sediment is most favored by climates which are hot and alternately wet and dry as opposed to climates which are either constantly cool or constantly wet or constantly dry.

Origin of Later Algonkian Rocks.—The question under this heading is as to whether the rocks of the Belt series of Montana, the Grand Canyon series of Arizona, the Llano series of Texas, the Avalon series of Newfoundland, and more doubtfully the Keweenawan series of the Lake Superior region, are of marine or of terrestrial origin.

By referring to the accompanying map of the later Algonkian rocks we find that all of the known areas are within the limits of the outline of the continental platform, even those of Newfoundland and Nova Scotia being 150 miles within that platform. With this in view I will first call attention to the origin of the great series of Tertiary terrestrial non-marine sediments in the western section of the continent, for the solution of that problem has a most important bearing on the probable origin of the Algonkian sediments.