OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 435 



of the Appalachian chain, where they are very common. They are to be seen 

 abundantly in the Jura, and in the exterior hills of the Alps. They abound, too, 

 in the undulated palaeozoic region of Southern Belgium, and are a marked fea- 

 ture in the coal-basins of that country.* 



These flexures prevail wherever the forces that disturbed the crust were neither 

 excessively intense nor very feeble. They usually hold an intermediate position 

 geographically, answering to the middle place they occupy as respects energy of 

 undulation between the groups of flat symmetrical waves, and those which are 

 closely folded, to the description of which I next proceed. Almost invariably, 

 those of a simply undulated tract, exhibit their steeper slopes directed all to one 

 quarter. 



Folded Flexures. 



This third and remaining class consists of flexures in which there is an inver- 

 sion or doubling under of the steeper side of each convex curve or wave. When 

 this structure is at a maximum, the folding back, downwards, of each convex 

 or anticlinal arch amounts almost to a parallelism of the two branches or sides of 

 these curves ; and where there are several such foldings, alternately convex and 

 concave, the strata may be said to be crimped or plicated into one dip, though 

 the entire change of inclination through which the inverted portions have 

 been bent, amounts to the supplement of the angle of the dip or the difference be- 

 tween the apparent dip and 180°. It is a necessary feature of all such folded 

 flexures, that the approximately parallel sides of the folds dip obliquely and not 

 perpendicularly to the horizon. They are, therefore, but exaggerated instances 

 of the class of normal flexures, or those where one branch of the curve is steeper 

 than the opposite. As in the case of the normal flexures, the more incurved sides 

 of these folded waves all look the same way. 



Axis Planes. 

 It is convenient, for the purpose of expressing the kind of flexure, its degree, 

 and its direction, to make reference to the geometric planes which bisect or equally 

 divide the anticlinal and synclinal bends. These imaginary planes we have called 

 the axis planes of the undulations, being those which include all the horizontal 

 lines or axes round which the individual concentric strata have bent in the act of 

 undulating or folding. In the first- described class of flexures, or those of symme- 

 trical curvature, each anticlinal and synclinal axis plane is necessarily perpendi- 

 cular to the horizon. In the second class, or the normal flexures, these axis planes 

 are necessarily not perpendicular, but steeply inclined to the horizon, and their 

 deviation from the perpendicular is in proportion to the difference of inclination, 



* See Dumont's Memoir sur les Terrains Ardennais et Rhenan, &c. 

 VOL. XXI. PART III. 6 B 



