3388 Prof. C. Lapworth—The Secret of the Highlands. 
The arch-limb, which has given way in an upward direction to 
the lateral pressure upon the strata, now overlaps above, while the 
trough-limb thrusts itself downwards below in the diametrically op- 
posite direction. Both arch-limb and trough-limb suffer from the 
general horizontal side-thrust of the earth-crust only; but the inter- 
mediate middle limb hes wedged in between two masses of strata 
moving in opposite directions, at the same time that it is enormously 
burdened by the weight of the superimposed arch-limb and arch- 
core, while, in addition, it is squeezed by a component of the 
general side-thrust. As a natural result, the middle limb suffers a 
mechanical action, which may most fitly be termed a mangling or 
rolling-out-action, and must thereby become longer. 
As the strata enveloped in the fold gradually give way to the 
lateral compressing forces, the beds forming the arch-limb can only 
move forward at the crest of the fold (which has a sharp curvature), 
by rolling themselves over in a downward direction towards the 
squeezed middle limb. ‘There constantly takes place, therefore, an 
over-rolling movement at the crest, as the folding goeson. Having, 
in this manner, arrived in the region of the middle limb, the various 
strata formerly belonging to the arch-limb are there crushed and 
rolled out. 
A corresponding and symmetrical movement takes place in the 
trough-curve. The end of the trough-curve is dragged upward in 
a rolling movement towards the middie limb, where its strata are 
crushed, flattened out, and extended. 
On the convex or outside parts of the arch and trough-curves there 
is a continual tendency of the particles to the constantly lengthening 
middle-limb: on the concave or inside parts of these curves there 
is a corresponding movement from the centre of the middle limb 
towards the apices of the curves. The geologically younger beds 
of the folding system (i.e. the parts within the arch and trough- 
axes) move downwards below the under side of the middle limb; 
the geologically older beds glide upwards above the upper side of 
the middle limb. But these changes of position of the particles can 
never proceed, however, so quickly as the advancing movement of 
the over-riding and underthrust masses, for they have merely been 
rolled and dragged along by the latter. 
Through the rolling-out process described above, the mass of strata 
dragged over the arch-bend grows thinner and thinner the greater 
its distance becomes from the overturning brow of the arch. In the 
same way, the lower portion of the middle limb, which feeds itself 
from the trough-limb by dragging particles upwards, rolls out 
thinner and thinner the farther it becomes removed from the trough- 
bend. Between both these regions the precise spot where the middle 
limb is at its weakest marks the special point where, in the original 
formation of the inclined fold, the curve between arch and trough 
originally changed direction, and which has therefore experienced 
for the longest time the rolling and dragging-out action in both 
directions. | 
(As a general rule the middle limb is rolled out uniformly, bu 
