364 DYNAMICAL GEOLOGY. 



range ; (3) the axis, is usually nearly straight, but sometimes bends around 

 through a large arc ; (4) instead of one flexure for the range, or parallel 

 flexures of like length, there is generally a succession along the mountain 

 region, one rising near where another ends, making overlapping series. 

 There is no crumpling of the beds, and no long intervals of horizontal beds 

 alternating with the flexed. Some single flexures are 80 to 100 miles long ; 

 and they vary in span from one mile or so to 20 or more. In the finer kinds 

 of rocks flexures occur of a few inches or less, which are like wrinkles on 

 the great rock-sheets. 



(5) The flexures have rarely the ridge line horizontal ; and, in adjoin- 

 ing flexures it often inclines in opposite directions, this being a mechanical 

 effect in the process of warping. 



Further (6), the axial plane of a flexure is seldom vertical, the opposite 

 slopes, in a transverse section, being unlike ; hence the flexures are mostly 

 unsymmetrical, even when not overthrust flexures (page 103). Again (7), the 

 flexures have the steeper side generally facing northwest, aioay from the Atlan- 

 tic Ocean ; at the same time they are by far the most numerous and close- 

 pressed, and most generally overthrust, in the eastern part of the range, or 

 the side toivard the ocean. The mountains have consequently a front-and-rear 

 structure, the front side facing the ocean. 



This flexing of rocks to such depths appears less incredible when it is 

 noted (a) that the strata so treated were generally those of sedimentary 

 formations ; (6) that they were, for the most part, only partially consoli- 

 dated, the limestones excepted ; (c) that all the rocks contained much 

 moisture, and had their cohesion diminished thereby ; (d) that as the move- 

 ment proceeded, heat was being generated by friction, which, if low in 

 degree, made siliceous solutions that would diminish friction by the dissolv- 

 ing action, and, if high, produced superheated vapor and a general softening 

 of the flexing masses. 



Again (8), great upthrust faults, with displacements 10,000 to 20,000 

 feet or more, exist in the region of flexed rocks, and especially where the 

 flexures are overthrust and close pressed ; and they are sometimes, if not 

 generally, flexure-faults, with the thrust westward along the flank plane of 

 the overthrust flexure. Professors W. B. and H. D. Eogers, in their ad- 

 mirable paper on the Appalachians, observe that " the passage of an inverted 

 fold into a fault is of common occurrence," and that some flexure-faults 

 have, " in southwestern Virginia, a length of about 100 miles." They 

 always occur on the northwest side of the flexure, as in the following 

 figures taken from two of their sections ; and they begin, say these authors, 

 with the thinning, or ''disappearance of one or more of the groups of 

 softened strata lying immediately to the northwest of the more massive 

 beds." " The dislocation increases as it is followed along, until finally the 

 lower beds (II) of the Lower Silurian are found resting directly on rocks 

 of the Carboniferous series (X, XI)." These two sections are from the 

 same fault, the first near its place of beginning, and the second, where the 



