Page:The American Cyclopædia (1879) Volume XVI.djvu/139

Rh outliers, is 100 m., in Pennsylvania and Maryland. The height of the plain at the base is 500 ft. in New England, and becomes 1,200 ft. S. of Virginia; the W. base in Virginia and Tennessee is from 1,000 to 2,000 ft. high. The Appalachians make their nearest approach to the sea in the Highlands on the Hudson, which are about 30 m. from Long Island sound; in the south the distance from the coast is 200 m. The Atlantic slope, between the Appalachians and the ocean, is in general hilly, with level tracts near the shore, particularly in the south. The great central district between the two mountain systems is a region of prairies and plains, sloping from each toward the Mississippi river, with a gentle southern decline to the gulf of Mexico. A portion in the northeast slopes toward the great lakes, and the basin of the Red river of the North toward the north. The elevation at the base of the Rocky mountains in Montana is 4,091 ft.; at the mouth of the Yellowstone river, on the border of Montana and Dakota, 2,010 ft.; at Denver, Colorado, 5,267 ft.; of the Llano Estacado in Texas and New Mexico, 3,200 to 4,700 ft.; of the source of the Mississippi in Minnesota, 1,680 ft.—Some details of the distribution of the great geological formations over the territory of the Union, and the relations of these to its geography, have already been given in the article, vol. vii., p. 695. It is there stated that the eozoic formations bearing the names of Laurentian, Huronian, Montalban, and Norian make up the Atlantic belt of the Appalachians, extending from E. Canada through New England and E. New York to N. E. Alabama. To these eozoic groups belong the White mountains, the Green mountains, the Adirondacks, the Highlands of New York and New Jersey, the South mountain of Pennsylvania, and its continuation south of the Potomac, the Blue Ridge. The lower levels of E. New England are also, with some exceptions, occupied by eozoic rocks, and the same is true of a broad belt of rolling country between the E. base of the Blue Ridge and the low lands of the coast. In addition to what has been said with regard to the distribution of the various formations over this area, it may be noted that rocks of the Laurentian age extend from the Hudson to the Schuylkill, while further southward Huronian and Montalban rocks prevail, including however a belt of Laurentian in Virginia. Westward from the Adirondacks eozoic rocks, embracing the four great types already mentioned, extend through Canada to N. Michigan and Wisconsin, while southward they reappear in the Ozark mountains of Arkansas, and also in small areas in Missouri. In the Rocky mountains eozoic rocks appear which seem to be identical in their character with those of the Appalachians, but have not yet been critically studied. At the W. base of the Green mountains, and thence extending along the W. flank of the South mountain and the Blue Ridge as far as Georgia, is a series of

rocks to which Prof. Emmons gave the name of the Taconic system. He described them as having a total thickness of about 20,000 ft. and consisting of an upper and a lower division; the latter consisting of sandstones and quartzites, followed by a great mass of limestones interstratified with and overlaid by argillaceous and magnesian schists, and destitute of fossils; while the upper division, including sandstones, slates, and limestones, contained a palæozoic fauna supposed by him to be older than that of the Potsdam and calciferous of New York, which latter were declared to overlie unconformably the Taconic system. These views were opposed by most American geologists, and chiefly by Mather, H. D. Rogers, and Logan. According to these authorities, the whole Taconic system represented in a modified condition the middle and upper Cambrian rocks, from the Potsdam to the Oneida. (See, tabular view, vol. vii., p. 694.) Later researches have confirmed the views of Emmons as to the antiquity of a portion of the fauna of the upper Taconic rocks, while the lower Taconic may correspond to the lower Cambrian of Europe, or perhaps to a still earlier period more closely related to the eozoic rocks already noticed. These rocks are important as making up the chief part of the floor of the great Appalachian valley from Lake Champlain to Georgia, a region remarkable for fertility of soil and for the great deposits of brown hematite (limonite) iron ore which belong to the lower Taconic strata. The American subdivisions of the middle and upper Cambrian rocks, from the Potsdam to the summit of the Loraine (or Hudson river) shales, appear in their characteristic forms in the valleys of the Mohawk and the St. Lawrence. From this great plain around the base of the Adirondacks they extend westward to the Mississippi valley, and thence southward as far as Texas. They are also found at intervals along, the eastern border of the palæozoic basin as far as Tennessee, but their precise relations to the Taconic rocks along this line are still involved in discussion. The base of the next great palæozoic division, the Silurian proper, is the Oneida, which rests unconformably upon the preceding, and, being a strong and massive sandstone or conglomerate, gives rise to a conspicuous ridge along the eastern border of the basin. It forms the Shawangunk mountains of S. E. New York and the Kittatinny mountain of Pennsylvania, stretching thence southward and bounding the great Appalachian valley on its N. and W. side, while the crystalline rocks of the South mountain and the Blue Ridge enclose it on the south and east. The whole eastern portion of the great palæozoic basin has been much disturbed by undulations of the strata having a general N. E. and S. W. direction, often complicated by fractures with great vertical displacements of the strata, which may be described as upthrows on the N. W. side of the faults. This disturbed region includes the whole of the palæozoic