379 



2. The flexures, when the undulated belt is broad, exist in groups 

 of waves, and the parallelism is generally more perfect between the 

 members of a given group than between one group and another. 



3. Usually where the zone of undulated strata is extensive, there 

 are several orders of waves, as regards their dimensions, the se- 

 condary or lesser classes constituting as it were ripples on the slopes 

 and summits of the primary or larger. These minor flexures, or 

 subordinate rolls, are themselves parallel, but not always necessarily 

 parallel with the principal waves upon which they lie. 



Form and Gradation of the Waves. 



Three essential varieties of form prevail among the great flexures 

 of the earth's crust. 1. The most simple is that of a symmetrical 

 wave, or one where the convex (anticlinal) or concave {synclinal) 

 curve is of equal flexure upon both slopes. This form belongs 

 chiefly to the flatter and broader waves, and when met with 

 among those of steeply-inclined sides, is apt to be accompanied by 

 an angular bending or even partial dislocation at the anticlinal or 

 synclinal axis. 2. A second prevailing form is where one side of the 

 wave is visibly steeper than the other. This is the normal type of 

 flexure in the Appalachian chain, in the Jura mountains of Switzer- 

 land, and in the undulated zone of Belgium and the Rhenish Pro- 

 vinces. 3. The third class of flexures embraces those which exhibit 

 an inversion or folding under of the most bent slopes of the several 

 waves. This doubling under frequently amounts to an almost per- 

 fect parallelism of the two sides of the flexures. In such cases 

 where the alternate convex and concave bendings are numerous, and 

 the whole belt is closely plicated, a transverse section presents the 

 puzzling phenomenon of strata of diff^erent ages dipping in one direc- 

 tion, in parallel, seemingly conformable superposition, the newer rocks 

 underlying the older ones as frequently as they overlie them. 



Conceiving a series of imaginary geometric planes to bisect the 

 successive anticlinal and synclinal bends in a belt of undulated strata, 

 these axis planes, as they may be called, are, in the case of the sym- 

 metrical class of waves, necessarily perpendicular ; but, in the other 

 tw^o classes, they are inclined to the horizon, and their dip or incli- 

 nation is flatter as tho waves approach the form of most extreme 

 folding with inversion. In many districts, as along the south-eastern 



