OF THE APPALACHIAN CHAIN. 



475 



comparison, to prevail in many of the disturbed regions of other 

 countries, and among sti'ata of all geological dates, an exposition 

 of their laws cannot be uninteresting at this time, when every 

 question connected with the elevation of the earth's crust, is 

 receiving so generally the attention of geologists. 



To render our details intelligible in the absence of a geological 

 map, we must first enter upon a brief geographical description of 

 the extensive zone of country wiiere these structural conditions 

 exist. Such a preliminary sketch is the more essential, since, in 

 no region yet described, does the topogi-aphy or physical relief of 

 the surface, afford as accm-ate an index to the positions and rela- 

 tions of the strata, and to the movements by which they have 

 been uplifted. 



The Appalachian chain rises in the form of a broad belt of 

 mountain ridges east of the St. Lawi'ence, in the northern part of 

 New En-gland, and, taking a southwesterly course, terminates in 

 Alabama. Its total length is about one thousand three hundred 

 miles, and its gi-eatest breadth about one hundred, if we exclude 

 from this description the high insulated tracts of the White Moun- 

 tains in New Hampshire, and that west of Lake Champlain, in 

 Nev/ York. From the northern border of Vermont, the main 

 chain gradually expands in width to the region of the Juniata and 

 Potomac rivers, beyond which, in its progi-ess to the southwest, 

 it slowly and steadily contracts to its termination. While the 

 gi-eat chains of many countries contain a principal central moun- 

 tain axis, to which all the minor ranges more or less conform, this 

 system consists of a broad zone of almost innumerable parallel 

 ridges of nearly equal average height. These seldom reach an 

 elevation of four thousand feet above the sea ; nor, if we except 

 the gi-eat eastern range, the Blue ridge, do they often rise more 

 than two thousand feet from the level of the adjoining valleys, 

 the more usual height being from eight hundred to one thousand 

 five hundred feet. The general plain, supporting this broad 

 mountain beh, gradually decUnes in level from the head waters 

 of the Holston and Clinch rivers, in Virginia, towards both 

 extremities. 



The characteristic features of the Appalachian ridges, are their 



