842 Prof. C. Lapworth—The Secret of the Highlands. 
undergoing deformation lie near the surface, and, owing to the 
absence of overlying rocks, they have comparative freedom of move- 
ment in a vertical direction. They consequently yield to the tan- 
gential crust stress by looping themselves up in broad masses into 
vast arches, the crests of which are directed obliquely outwards 
perpendicularly to the direction of local thrust; and the phenomena 
we have described affect the entire mass of strata enveloped in each 
fold, considered as a whole. 
But at great depths beneath the surface, where the strata are over- 
burdened by masses of superimposed rocks (and also in the cores 
and curves of the larger arches and folds themselves, where they are 
surrounded by enveloping rock-masses), the deforming strata have 
no longer the same freedom of vertical movement. Hence we may 
infer as a natural result, that the amplitude of the rock folds grows 
less and less in proportion as the weight of overlying material 
increases. The various stages of rock deformation under lateral 
pressure (folds, overfolds, overfaults, and overthrusts), under these 
new conditions are theoretically of the same kind as before, and take 
place in the same sequence, but are of rapidly diminishing degree. 
Asa typical term in the descending series, we select. that theoretical 
case in which we shall suppose that the tangential thrust acts upon 
one stratum only. 
As the general rock-mass in which it is imbedded gives way as 
a whole to the grand horizontal stress, the stratum is wrinkled up 
into a series of corrugations, the magnitude of which depend upon 
(a) the weight of the overlying rocks, (0) the thickness of the 
stratum itself, and (c) its comparative hardness and density. 
As the overlying mass gives way and rises above each individual 
bed becomes in this way crumpled inwards upon itself into a set of 
minute and more or less symmetrical folds, each of which consists 
as before of an upward curve or arch, and a downward curve or 
trough, and an intermediate neutral wall or common limb, the axial 
planes of all three being approximately parallel, and inclined in that 
special direction in which the general mass of strata is giving way. 
As the process goes on, each bed may retain its individuality, but 
must of necessity become greatly increased in apparent thickness, — 
viz. to an extent which is measured by the amplitude of its folds. 
As the pressure increases, the several stages of rock-deformation 
already described here follow each other in their natural sequence. 
The miniature normal folds pass into overfolds with attenuated 
middle limb; next into overfolds with insignificant or fragmentary 
common wall, till finally the latter may become obliterated altogether 
and become replaced by a plane of dislocation, the roof and floor of 
each miniature overfold resting at once upon each other in regular 
and conformable sequence. 
During this process those parts of the stratum forming the two 
side limbs have undergone compression and condensation: those in 
the neighbourhood of the common wall have been submitted to 
twisting and elongation. In all the later stages of its deformation, 
therefore, the bed is made up of wedges of strengthened material, 
