Page:The Journal of geology (IA journalofgeology11893univers).pdf/224
mechanical and chemical deposits in the Cordilleran sea after the middle Cambrian subsidence:
| Wasatch. | Central Nevada. | Southwest Nevada. | Montana. | Alberta. | |
|---|---|---|---|---|---|
| Mechanical Sediment, | 10,000 | 7,500 | 2,500 | 1,000 | 4,600 |
| Chemical Sediment, | 10,400 | 15,150 | 13,000 | 4,000 | 15,000 |
| Ratio | 1/1 | 1/2 | 1/5 | 1/4 | 1/3 |
If an average is taken of the mechanical sediment deposited subsequent to the close of middle Cambrian time, it will be found to be about 5,000 feet for the entire area, which, I think, does away with any necessity to assume an additional hypothetical land area for the source of the mechanical sediment. The fine sand composing the quartzites and the silt forming the shales, as well as the fine conglomerate of later deposits, were derived from the adjoining land areas, and, in all probability, currents swept through from the ocean to the south or north, distributing the mud and sand contributed from the rivers and streams along the shores.
Chemical Sediments.—The present supply of the carbonate of lime, silica, etc., contained in sea-water is derived from waters poured into the sea by rivers and streams. The Cordilleran sea undoubtedly received a large contribution from the adjoining land areas, but a considerable amount was possibly derived from an oceanic current that circulated through it as the southern equatorial current of the Atlantic now sweeps through the Caribbean. From the vast deposits of carbonate of lime it might be assumed, a priori, that the waters of a Mississippi or Amazon were poured into it, but there is not any evidence of the existence of such a river, although the tributary area may have been very large in Cambrian and Carboniferous time, if the drainage of the country west of Hudson's Bay was to the westward.
Conditions of Deposition.—With free communication into the open ocean on the south, and probably on the north, during most of Paleozoic time strong currents must have circulated through the Cordilleran sea. The broad distribution of