Prof. CO. Lapworth—The Secret of the Highlands. 348 
whose thickened extremities are turned in opposite directions, and 
these wedges are separated from each other by planes of weakness, 
or of actual dislocation. 
It is needless perhaps to follow this process farther. It is clear 
that in all its later stages the bed will be found to have undergone 
a peculiar change in its dimensions, having been greatly compressed 
in one direction, and elongated in another. It has given way 
under the lateral thrust by wrinkling itself up into folds whose 
axes are perpendicular to the direction of local pressure; and the 
entire mass is finally traversed by a series of planes of structural 
weakness (fold-faults) coincident with the axes of these contortions. 
In other words, we have before us simply the gradual development 
of that special type of schistosity, which is correctly identified by 
Sorby in his classical paper * with true slaty cleavage. 
From the conditions of the case all the phenomena of corrugation 
must be attended with great local irregularities, and may be more or 
less incapable of being referred to their proper type. But the various 
stages of the process of deformation ought fairly to be identified by 
the gradual disappearance of the miniature arch and trough-curves, 
and the final obliteration of the middle wall, and its replacement by 
a plane of cleavage. Some of the stages appear to be illustrated in 
the published figures indicated below.? In the earlier stages the 
corrugation is easily recognizable as such. In the intermediate 
stages we seem to have that interesting appearance which has been 
interpreted as false contorted bedding. In some of the later stages 
we find the thickened stratum traversed by those oblique laminations, 
which are usually regarded as affording unequivocal evidences of the 
false-bedded nature of the metamorphic schists. 
Deformation of mountain folds. 
If the causes and results of overfolding of rocks under tangential 
thrust are correctly laid down in the preceding paragraphs, in so far 
as they affect the groups of strata involved in the minor folds upon 
the flanks of mountain chains, they must, theoretically, be equally 
true of the grander masses of strata enveloped in the regional earth- 
waves, each of which gives origin to a single chain in a mountain 
system (Plate VIII., Fig. 7). When these mighty arches and troughs 
are brought closer together by the great crust-creep, the arches must 
1 On the Origin of Slaty Cleavage, by H. C. Sorby, F.G.S., etc., Edinburgh 
New Philosophical Journal, July, 1853. 
2 Compare :— 
(a) Corrugations only—ruge continuous. 
Ramsay, Geol. N. Wales, 1881, p. 239, fig. 96. 
Geikie, Handbook Geology, 1882, p. 520, fig. 240. 
(6) Corrugation with diminished middle wall. 
Ramsay, Ibid. p. 231, figs. 78 and 79. 
Geikie, Ibid. 120, fig. 19. 
(c) Corrugation with dislocation planes (‘‘ contorted false bedding ’’). 
Geikie, Ibid. p. 479, fig. 189. 
Lyell’s Students’ Elements, ed.’ 1871, fig. 628. 
(d) Overfolding and faulting of ruge. 
- (e) Overthrust, Bonney, Quart. Journ. Geol. Soc. 1883, pl. 39, fig. 2 
