340 Prof. C. Lapworth—The Secret of the Highlands. 
series of forms, each successive member of which may arise naturally 
from the preceding merely by an increase in the force of compres- 
sion. These overfolds may be thus distinguished :— 
(a) An overfold, originating by the overthrow of a central limb 
previously limited by two differently directed curves has three 
limbs of about equal strength. 
(6) An overfold with gradual development of a middle limb, 
which has originated in the over-rolling of the beds at the apices 
of the curves, has a greatly reduced middle limb. 
(c) The overfold which has originated by the shearing off of the 
arch portion from the trough portion, and upthrust of the former 
over the latter, has either (c') a middle limb squeezed very thin or 
else (c’) no middle limb at all. We find in this last type the 
two following cases :— 
(c’.) The arch portion of the middle limb is missing, but the 
trough portion is still present. 
(c') The entire middle limb is wanting. 
The arch limb and trough limb of an overfold merely undergo side- 
thrust ; the middle limb suffers stretching, rolling out, and squeezing. 
The deeper they lie below the surface the greater is the ductility 
of the strata. In overfolds of vast extent the arch limb being nearer 
the surface is more rigid, the trough limb, being buried under more 
than double the burden, is more ductile. It often breaks up there- 
fore under compression into a complete series of secondary folds. 
The position of these minor folds is prescribed to them by the limits 
of the primary trough of the overfold, for the whole space is filled 
up, and yielding is only possible in those directions in which the 
whole mass gives way. From this results the frequent harmonic 
folding of the trough limb. 
Such is the theory of the gradual development of reflexed folds 
as laid down by Heim in his great work; and no one who has 
carefully studied in detail the appearances of the strata enveloped in 
the numberless overfolds in convoluted rocks, can possibly doubt its 
general correctness. Accepting, therefore, this fundamental theory 
as it stands, we obtain the following additional principles of moun- 
tain structure : 
11. In the process of the folding of the beds in an overfold (Plate 
Vill. Figs. 1—5), while the particles in the roof and floor of the 
overfold become crushed more closely together, those in the middle 
limb are subjected to gradually increasing extension. This gives in 
orderly sequence the successive formation of 
(1) An overfold composed of three nearly equal limbs. 
(2) An overfold with insignificant middle limb. 
(3) An overfold with local relics of a middle limb. 
(4) An overfold with a plane of dislocation in lieu of the middle 
limb (fold-fault). 
(5) As the pressure increases, the strata shear along the plane of 
dislocation; the axial plane of the fold (plexal axis) becomes 
the plane of an inverted fault, and the arch portion of the 
fold slides upwards over the trough portion until equilibrium 
is restored (upthrust). ; 
