OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 467 



General Resume. 



1 . Wave-like form of all belts of uplifted strata. 



a. It is a general fact that strata dip in curved and not in straight planes. 



b. Wherever wide areas of the crust have been elevated or depressed from the 

 level at which their strata were deposited, these strata will be found, except 

 where their dip is disordered by crust dislocations, to constitute, in their vary- 

 ing angles of dip, one or more wide regular curves. 



2. Parallelism of crust undulations. 



a. It is another general fact, that these undulations of the strata are in the form 

 of long parallel waves, resembling much those great continuous billows called 

 in dynamics waves of translation, and by seamen rollers. 



3. Relations of flexures. 



a. Parallelism of the waves to the general trend of the part of the mountain sys- 

 tem to which they belong, and especially to its chief igneous axis. 



b. Parallelism of flexures extends not only to adjacent individual waves, but to 

 contiguous groups, and is as true of curvilinear as of straight. 



c. The waves of the strata are generally of two or three grades of magnitude, as 

 respects their length, height, and amplitude, and while those of the same grade 

 are parallel, the different grades are not necessarily so. 



4. Laws of form and gradation of waves. 



There are three characteristic forms of crust waves ; symmetrical flexures equally 

 steep on the two slopes ; normal flexures, curving more rapidly on one side 

 than on the other ; and folded flexures, or those with a doubling under of their 

 more incurved slopes, and among which the steepest slopes are generally directed 

 to the same quarter. 



The geometric planes bisecting the anticlinal and synclinal bends of the strata, 

 here called axis planes, are nearly perpendicular in the symmetrical waves, but 

 inclined in the other two classes, dipping at the lowest angle in the folded 

 flexures. In many belts the plication is such as to amount to parallelism of 

 all the inverted to the uninverted sides of the waves. 



Some waves are straight, some curvilinear and crescent shaped, and many of 

 them extremely regular, changing their trend 40° or even 50°. The curvilinear 

 ones convex from the disturbed sides of the zones, are generally more regular 

 than those which are convex towards them. 



VOL. XXI. PART III. 6 K 



