5 



OF THE MORE DISTURBED ZONES OF THE EARTH S CRUST. 433 



impressed with its marked resemblance in all the essential features of the un- 

 dulations, both as respects the typical forms of the individual waves, and the 

 grouping and gradation of the several sets of waves, to the flexures character- 

 istic of the Appalachian chain of America. I was particularly convinced of 

 this resemblance upon examining, in the summer of 1848, the structure of 

 the Jura chain of Switzerland ;* and scarcely less struck with the agreement I 

 noticed between the phenomena on the borders of the Alps, especially in the 

 Bernese Oberland, and the features which distinguish the most corrugated 

 tracts at the south-eastern base of the chain of the Appalachians. 



Relations of Flexures to each other. 



If we regard now the flexures which constitute any great undulated or cor- 

 rugated belt of strata, we shall find that these display the following laws or ge- 

 neral facts of relationship : — 



Parallelism. 



1. When seen in their simplicity, or undisguised by cross breaks and undula- 

 tions, those of a particular district show a remarkable degree of mutual parallel- 

 ism. Not only are they parallel to each other, but to the general trend of 

 the portion of the mountain system to which they belong, and especially to its 

 chief igneous axes, where it possesses such. 



2. The flexures or waves, where the undulated zones are wide and complex, 

 occur in groups or lesser belts ; those constituting such subordinate series ob- 

 serving the law of parallelism still more strictly than group does towards group. 

 This remarkable parallelism of the adjacent flexures in an undulated region be- 

 longs not only to those waves and groups of waves which are rectilinear in their 

 crests, but to such as curve even very considerably in their lineation. Nowhere, 

 perhaps, is the constancy of this law so well displayed as in the Appalachians. 

 This great mountain zone of the United States and Canada, about 1500 miles in 

 length, and more than 150 in its maximum breadth, consists longitudinally of eleven 

 different sections, six of which are straight, three curvilinear, and convex towards 

 the north-west, and two also curvilinear, but convex towards the south-east. 

 Three of the straight sections have an approximately east and west trend, and the 

 other three an approximately north and south course. Notwithstanding the great 

 windings in the direction of the chain thus indicated, it is remarkable that each 

 division or segment of it, whether straight or curved, is made up of crust- waves 

 and groups of waves, which are essentially in mutual parallelism ; and wherever 

 a seeming exception to this rule presents itself, as on the Upper Juniata in Penn- 



* See Abstract of Communication to American Association for Advancement of Science, Cam- 

 bridge, Mass., March 1849, p. 113. 



