380 



side of the Appalachians, and on both flanks of the Alps, these axis 

 planes, or what is the same thing, the foldings of the rocks, incline at 

 a very low angle, implying an excessive amount of horizontal move- 

 ment at the time the strata were thus plicated and packed together. 

 This parallel reduplication of strata is usually attended by more or 

 less metamorphism, amounting to that change of internal structure 

 which is denominated cleavage ; and the cleavage planes, frequently 

 more conspicuous than the original planes of sedimentation, serve 

 still further to conceal the flexures, and disguise the true order of 

 superposition of the rocks. 



Waves of the Crust both Straight and Curvilinear. 



In the much corrugated belts, the crust waves are both straight 

 and curvilinear. In the Appalachians there are groups of both 

 these classes, retaining their special features throughout their entire 

 length, which, in some instances, exceeds 100 miles. Some of the 

 crescent-shaped waves present their convex curvature towards the 

 region of maximum dislocation and metamorphism, while other 

 groups are concave toward the same quarter. These different sys- 

 tems of waves seem to have been generated some of them from 

 straight, others from curvilinear fractures in the earth's crust. 



The Appalachian chain, regarded in the light of a long zone, or 

 chain of groups of parallel straight and curving waves, consists of 

 eleven sections, six of which are straight and five curvilinear, three 

 of the latter form being convex towards the N.W., and two convex 

 towards the S.E., the whole zone having a length of 1500, and a 

 maximum breadth of 150 miles. Certain of the straight divisions 

 have their anticlinal axes, or the crest lines of the undulations trend- 

 ing N. 15° E. ; other divisions, theirs trending N. 70" E., while 

 some of the curving sections of the chain show a deflection in the di- 

 rection of their individual axes of as much as 40". Indeed, in par- 

 ticular instances, the change of trend amounts to as much as 60°. 

 So remarkable a bending without disruption, of groups of parallel 

 anticlinals seems incompatible with the inferences of some eminent 

 geologists, who conceive that there prevails a general relation 

 throughout the globe between the directions of the lines and the 

 epochs, of crust elevation ; for we here find that the self-same axis, 

 generated throughout its whole length, not merely in one geolo- 



