916 PHYSIOGRAPHICAL GEOLOGY. [Book VIL. 
pass into the unsymmetrical type, then with gradually lessening 
slopes into the symmetrical, finally widening out and flattening 
into the plains. If we bisect the flexures in a section of such a 
plicated region we find that the lines of bisection or “axis-planes”’ 
- are vertical in the symmetrical folds, and gradually incline towards 
the more plicated ground at lessening angles.’ 
Fractures not infrequently occur along the axes of unsymmetrical 
and inverted flexures, the strata having snapped under the great 
tension, and one side (in the case of inverted flexures, usually the 
upper side) having been pushed over the other, sometimes with a 
vertical displacement of several thousand feet. It is along or 
parallel to the axes of plication, and therefore coincident with the 
general strike, that the great faults of a plicated region occur. As 
a rule dislocations are more easily traced among low grounds than 
among the mountains. One of the most remarkable and important 
faults in Europe, for example, is that which bounds the southern edge 
of the Belgian coal-field (p. 746). It can be traced across Belgium, 
has recently been detected in the Boulonnais, and may not impro- 
bably run beneath the Secondary and Tertiary rocks of the south of 
England. It is a remarkable fact that faults which have a vertical 
displacement of many thousands of feet produce little or no effect 
upon the surface. The great Belgian fault is crossed by the valleys 
of the Meuse and other northerly flowing streams. Yet so indis- 
tinctly is it marked in the Meuse valley that no one would suspect 
its existence from any peculiarity in the general form of the ground, 
and even an experienced geologist, until he had learned the structure 
of the district, would scarcely detect any fault at all. 
(e.) Alpine Type of Mountain Structuwre.—It is along a great 
mountain chain like the Alps that the most colossal crumplings of 
the terrestrial crust are to be seen. In approaching such a chain 
one or more minor ridges may be observed running on the whole 
parallel with it, as the Jura ridges flank the north side of the Alps, 
and the sub-Himalayan ridges follow the southern base of the Hima- 
layas. On the outer side of these ridges the strata may be flat or 
gently inclined. At first they undulate in broad gentle folds; but 
traced towards the mountains these folds become sharper and closer, 
their shorter sides fronting the plains, their longer slopes dipping 
in the opposite direction. This inward dip is often traceable along 
the flanks of the main chain of mountains, younger rocks seeming 
to underlie others of much older date. Along the north front of 
the Alps, for instance, the red molasse is overlaid by Eocene and 
older formations, ‘The inversions increase in magnitude till they 
reach such colossal dimensions as the double fold of the Glirnisch, 
where Triassic, Jurassic, and Cretaceous rocks have been thrown 
over above the Kocene flysch and nummulitic limestone (p. 518). 
In such vast crumplings it may happen that portions of older strata 
are caught in the folds of later formations, and some care may be 
' HI. D. Rogers, Trans, Roy, Soe, Edin, xxi. p. 434. 

