516 OF THE PHYSICAL STRUCTURE 



each other in an obtuse angle, but did not meet ; and that, in the 

 angle between them, there was afterwards formed another inter- 

 mediate belt of undulations. The extremities of these last waves, 

 encountering the flexures already formed in the adjoining straight 

 belts, w\)uld be obstructed and retarded in their progi-ess north- 

 westward ; but the middle portion of each billow, moving in a 

 tract as yet free from permanent axes, would meet with less im- 

 pediment, and advance with a higher velocity, so as to impart to 

 the whole of each axis a curvelinear form. It appears, moreover, 

 highly probable, that the fractures of the crust in the dislocated 

 district in the southeast, would themselves be more or less curvi- 

 linear in the vicinity of previously formed rents approaching each 

 other at an obtuse angle, and thus a tendency to that shape might 

 be primarily impressed on all the undulations taking their origin 

 in a region so circumstanced. 



On the other hand, in those sections of the chain where the 

 axes have a concave curvature northwestward^ and where there 

 usually exists less regularity in their sweep than in the convex 

 gi'oups, we may imagine that the lines of elevation of the two 

 adjacent straight belts, terminating nearer and nearer to each 

 other, as the axes receded towards the northwest, would soon 

 mutually interfere, and the undulations originating at the south- 

 east, in the space opposite the angle, would find their progress 

 northwestward more and more in^peded, as they advanced through 

 the narrowing area between the ends of the flexures previously 

 formed. By unequal and multiplied obstructions thus occasioned, 

 the regularity of the axes in the intermediate division would be 

 greatly impaired. 



There is a curious arrangement in echellon, which we notice 

 in many of the gi-oups of axes of the Delaware river or New Jer- 

 sey division, where, though individually nearly straight, they 

 change their strike more and more to the north as we advance 

 northeastward. This admits of a simple explanation, if we merely 

 suppose a portion of the flexures in the next straight belt on the 

 southwest to have been first produced, and these to have been 

 followed by those on the northeast, which occupy New Jersey 

 and the contiguous districts of New York, the undulations starting 



