502 OF THE PHYSICAL STKUCTURE 



the Potomac division, the sti-aight axis of Wills's creek moun- 

 tain, ninety miles in length, and also that of the Knobly moun- 

 tain, nearly a prolongation of the last, itself a hundred miles long. 

 To these we may add, for the Holston region, the straight axis of 

 \Yolf creek, and that of the Clinch mountain, the former of which 

 is about one hundred miles, and the latter more than one hundred 

 and twenty miles in length. 



It is probable, that numerous axes of the folded or inverted 

 type, quite as extended, exist in the great valley, and the adjacent 

 belt of ridges on the southeast side of the chain, and we have al- 

 ready seen, that where some of the steep normal and inverted 

 flexures pass into dislocations, they have a length even exceeding 

 that of any of the axes above referred to. If we turn to the more 

 depressed normal axes of the western coal region, we shall find, 

 that that which lies next northwest of the Potomac basin, is at 

 least seventy miles long, that of the Negro mountain ninety miles, 

 that of Laurel hill at least ninety miles, and that of Chestnut 

 ridge, or West Laurel hill, more than one hundred miles in 

 length ; and our geological maps will exhibit, in other less well- 

 known portions of the same belt, a series of similar obtuse flexures, 

 of even still more extended length. 



7th. Of the curving' of axes. It is needless to add much to what 

 has already been said or infen*ed concerning the horizontal inflec- 

 tion of the axes in some groups, since the changes of stiike men- 

 tioned, while tracing the great divisions of the chain, involve a 

 parallel bending of all the principal and most influential flexures 

 individually. Considering the enormous extent of warping, which 

 the crust must have undergone, and which we can infer that it did 

 undergo, from the evidence afforded in the lesser, or secondary 

 flexures, and also from the nature of the faults, the prevailing 

 continuity and graceful, ciu'ving oufline, witnessed in many of the 

 inflected axes, are truly remarkable. There are cases, as in that of 

 the Jack's mountain flexure, where a continuous axis sweeps for 

 ninety miles, to undergo a change of strike of as much as forty- 

 five degrees, without once talcing on a serpentine or contrary in- 

 curvation, or manifesting any considerable inequality in the 

 bending. Instances of such extraordinary length and regularity, 



