Prof. C. Lapworth—The Secret of the Highlands. 339 
this is not invariably the case. It is sometimes torn into individual 
shreds, between which the strata of the arch-core and those belong- 
ing to the trough-core touch each other.) 
Whilst in the first type (a) of inclined fold described, a piece of 
strata corresponding with the width between the trough- limb and 
the arch-limb, is predestined from the very beginning to be the latet 
middle-limb of the inclined fold ; in this second type (b), the middle 
limb originates in great measure by the gradual curving round and 
rolling out of pieces of the two side-limbs. During the infancy of 
the fold (6), the particles eventually composing the middle limb 
were mostly portions of the lateral limbs, and underwent a mechanical 
transformation in their gradual transition into the middle-limb. It 
is evident that the zones of strata which were thus dragged by the 
two side-limbs into the central limb have, when combined, a collec: 
tive breadth necessarily much inferior to that of the middle limb 
which has been rolled out of this material. 
In the first case (a) the original breadth of the compressed zone 
of earth-crust enveloped in the final fold corresponded to the sum 
total of the three limbs; but in the second case (0), it only corre- 
sponds to the sum of the two side- limbs, plus a fraction of the 
middle limb. 
(Although the process described above may be regarded as the 
normal mode of origin of the second type of inclined fold, it may 
nevertheless be developed very naturally out of the fold of the first 
type, if the compressing force be more than sufficient for the forma- 
tion of (a).) 
(c) Asathird member in the series, there is the case when the 
middle limb is no longer sufficiently fed by the lateral limbs, or 
when the folding and rolling out go too far, or are continued to such 
an extent that the movements taking place in opposite directions 
on both sides of the middle limb, as well as the opposite move- 
ments of arch portion (limb and core), and trough portion (limb and 
core), are of necessity compelled to concentrate themselves upon an 
intermediate layer of middle-limb of ever-diminishing thickness: 
Finally, they come of necessity into immediate contact, and when 
this takes place, the arch-portion and trough-portion of the overfold 
are completely sheared off from each other. Instead of the middle 
limb we have now a surface of dislocation ; and the arch portion of 
the fold thrusts itself over the trough portion in the form of a more 
or less rigid mass. 
In many cases this overthrusting effect is due to the relief of 
downward pressure caused by the erosion of the brow of the arch. As 
a general rule, the trough bend is much longer able to furnish fresh 
material to the central limb than the arch bend, so that we find 
many inclined folds, which in their trough portions are of the type (6), 
and in their arch portions of the type (c). In these folds, a central 
limb is still recognizable in the vicinity of the trough bend, but, 
in the vicinity of the overlapping crest, they have become a rigid 
overthrust mass, without a trace of central limb. 
We find consequently among inclined or reflexed folds a connected 
