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Ohn B Mcdonnell

rocks, strata, deposits, time, carboniferous, triassic and period

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OHN B. MCDONNELL, Editorial itaff of The Americana.

Geology.• There is great diversity in the geology of the country, a diversity that extends to the constituent rock formations, their distri bution, structural arrangements and contained mineral deposits. Every important rock system is represented in its stratigraphy by one or more members. The basal formations belong to the early Pre-Cambrian and consist of ancient igne ous rocks, like granite and various lavas, with highly metamorphosed sediments in the form of crystalline limestones, schists and quartzites. Such occur along the axis of the Appalachians from New England into Alabama, and in the Black Hills, Ozarks and Rocky Mountains; they are also developed on a larger scale in the Adi rondacks and in the Lake Superior district, where they constitute outlying areas of the so called °Laurentian shield° of Canada which has a surface of 2,000,000 square miles. These rocks are particularly rich in iron ores, but yield also various other valuable minerals. In the Lake Superior district the Keweenawawan beds of sandstones and intercalated lavas mark the closing stages of Pre-Cambrian time; they at tain an estimated thickness of 40,000 feet and are the sources of the great deposits of native copper that occur on Keweenaw Peninsula of Upper Michigan.

A long period of time elapsed in most places after the Pre-Cambrian until the first deposits of the Paleozoic group were formed, the hiatus being marked by a strong unconformity caused by erosion. The ensuing Cambrian strata rest upon the eroded and more or less upturned Pre Cambrian rocks and are generally to be distin guished by their less altered condition and wealth of fossils. The Cambrian, Ordovician and Silurian strata, the older members of the Paleozoic group, spread over extensive areas in the eastern United States, lying along the bor ders of the crystallines in successive overlap ping series. Altogether they have here a thick ness probably of seven or eight miles, mainly sandstones, shales and limestones. In the west they occur in smaller belts and isolated patches.

The valuable minerals are oil and gas in the Ordovician and Silurian, salt in the Silurian and iron ore represented by the beds of Clinton hematite which extend from central New York in interrupted outcrops all the way to northern Alabama.

The succeeding members of the Paleozoic in cluded in the Devonian and Carboniferous sys tems are largely sediments that accumulated in great thickness in the Appalachian belt and west as far as the Mississippi Valley, also to a lesser extent in the Rocky Mountain and Great Basin regions. Oil, gas and coal abound in these strata, the Carboniferous being a great coal making period the world over. With the close of Carboniferous time a great change took place in the eastern part of the country, by which the sediments along the Appalachian region were uplifted from the sea and folded into moun tains, which have ever since remained as a highland.

The Mesozoic strata are usually grouped into the Triassic, Jurassic and Cretaceous systems. A belt of Triassic rocks parallels the Atlantic Coast from North Carolina to New England. Sandstones, shales and trap are the chief rocks, the Palisades of the Hudson exemplifying one of the exposures of Triassic trap, a basic vol canic roc.k. Extensive exposures of Triassic strata are met with in the western interior, where they seem to have been accumulated in shallow lakes and as wind deposits. Jurassic rocks are limited in development mostly to the Pacific Coast. The Cretaceous was a period of extensive sedimentation, and its rocks spread in a wide belt from the Gulf of Mexico north ward across the central United States and Can ada to the Arctic Ocean. A thickness of four or five miles is attained by the sediments in the Rocky Mountain region. Valuable coal ures accompany the beds, second in importance only to those of the Carboniferous. Oil and gas also are found in the Cretaceous strata. Two mountain systems had their beginnings in Mesozoic time: the Sierra Nevadas were re raised in late Jurassic and the Rockies at the end of the Cretaceous period.

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