a 
a 
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 palzeozoic 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 intermedjate 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. 6B 
