668 THE POPULAR SCIENCE MONTHLY. 



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 chan- 

 neled by rivers and finally was mostly worn down to a broad 

 base-level or a moderately undulating expanse. Renewal of ele- 

 vation, occurring in the Jurassic period, was probably attended 

 with the remarkable overthrust faults, having apparently a maxi- 

 mum 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 low- 

 land 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 Appa- 

 lachian 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 Mount- 

 ains ; 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 mount- 

 ainous belt, extending nearly two thousand miles, is everywhere 

 characterized by overturned folds, and by intense metamorphism, 

 the sedimentary strata, originally shales, sandstones, and con- 

 glomerates, being changed to crystalline schists, gneiss, and gran- 

 ite. 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 



