Page:Popular Science Monthly Volume 39.djvu/686

668 the formation of the coal measures, the thick sediments that had been laid down in the subsiding eastern margin of the Palaeozoic ocean, which extended westward over the present basins of the Laurentian lakes and the Mississippi, were compressed into folds and raised to constitute a mountain mass one thousand miles long and seventy-five to one hundred miles wide, with probably much greater altitude than now. During Permian and Triassic time, according to Prof. W. M. Davis, this elevated area was channeled by rivers and finally was mostly worn down to a broad base-level or a moderately undulating expanse. Renewal of elevation, occurring in the Jurassic period, was probably attended with the remarkable overthrust faults, having apparently a maximum extent of about eleven miles of horizontal displacement, which have recently been studied out by C. W. Hayes, similar to the thrust-planes discovered by Peach and Home in northwestern Scotland. Another cycle of base-level erosion is shown by Davis to have extended from the Jurassic upheaval to the end of the Cretaceous period, reducing the Appalachian Mountains to a lowland tract, in part nearly flat and in part hilly, which he names the Schooley peneplain. This tract, almost a plain at the close of the Mesozoic era, was then a third time upheaved; and the present valleys of the Appalachian belt, divided by very long mountain ridges of uniform height, have been cut by river erosion during the Tertiary and Quaternary eras.

Closely associated with the foregoing are other folded groups and ranges of mountains, which Prof. C. H. Hitchcock has named the Atlantic mountain system, first raised as mountain masses in the Cambrian and Silurian periods, long before the great Appalachian revolution terminating the Coal period. In order from northeast to southwest, this system comprises low mountains in Newfoundland and in the eastern provinces of Canada, south of the St. Lawrence; the mountains of Maine; the White Mountains; the Green Mountains; the Hoosac and Taconic ranges; the Hudson highlands; Schooley's Mountain and other ranges in New Jersey; the South Mountain in Pennsylvania; the Blue Ridge in Virginia; and the Blue Ridge, the Stone Mountains, and the Iron, Bald, Smoky, and Unaka ranges in North Carolina. This mountainous belt, extending nearly two thousand miles, is everywhere characterized by overturned folds, and by intense metamorphism, the sedimentary strata, originally shales, sandstones, and conglomerates, being changed to crystalline schists, gneiss, and granite. Denudation of the Atlantic mountain system, and of lands stretching eastward over part of the present Atlantic Ocean area, supplied the deposits which were upheaved in the building of the Appalachian ranges.

A still older Laurentian mountain system, first upfolded in