to, lines of valley; while the synclinal folds have originated lines of ridge or hill.

FIG. 41.-Anticlines forming valleys; synclines forming hills.

But the observer as he extends his experience will find that not only have the rocks of the earth's crust been

folded in this equable and gentle way; but that they present proofs of much more intense movement. These

may be looked for in the vicinity of faults, as has already been pointed out. Sudden and and violent crumpling in the midst of comparatively undisturbed strata may be regarded as primâ facie indicative of the proximity of some dislocation. It will not however of itself be sufficient to prove the existence of a fault, for unexpected

local twists and severe plication sometimes occur where, though the rocks must have been subjected to intense lateral compression, they have not actually been fractured. On a small scale the most tumultuous contortion of soft strata may often be seen as the result of a landslip. In Fig. 43, for example, a set of dark shales lying under a

thick sandstone have been crushed up by slips of the heavy over-lying rock, yet the ruin has been so well concealed by vegetation that a careless observer might suppose the lower twisted beds to be much older than, and unconformably covered by, the upper horizontal strata.

There occur wide tracts of country where the underlying rocks have been so violently disturbed, that for miles they seem to be standing on end. In such cases it is usual to find some one prevalent direction of strike along which the vertical or highly inclined beds range themselves. And a careful examination will generally disclose

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FIG. 44.-Section of what at the surface might be mistaken for a continuous highly inclined series of strata, shown to consist of numerous anticlinal and synclinal folds. Gneissose rocks, Lock Quoich, Invernesshire,

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proofs that the strata really consist of many rapid folds, the same beds being repeated again and again. sequent extensive denudation has worn away the tops of the arches and produced a form of surface which may have little or no reference to the structure of the rocks below (Fig. 44).

Rocks contorted in this way are pretty sure to present cases of inverted folds, that is, the axes of the curves are not vertical but inclined. In Fig. 45, for example, the folds are all inclined in the same direction, so that in each of them one half of the curve has its strata turned

bottom uppermost. Inversions on a grand scale are to be seen in great mountain-chains like the Alps. The accompanying drawing (Fig. 46) represents a very remarkable example which occurs in the mass of the Glärnisch, one of the eastern Swiss Alps, as described by Dr.

Baltzer. The peak (Ruchen) reaches a height of 2,107 metres above the valley to the left of it (Klönthal). The folded rocks belong to the cretaceous system of the Alps.

FIG. 46.-Section of the grand inversions of strata in the Glärnisch Mountain. Eastern Alps.

Cleavage. By the powerful lateral pressure to which rocks have been subjected during their subsidence and contortion, their minute particles, which usually present one axis longer than the others, have been compelled to adjust themselves in the rock along the

line of least resistance; that is, with their longer axis perpendicular to the direction of the pressure. Mr. Sorby showed by ingenious experiments, that with suitable adjustments of pressure this re-arrangement could be imitated artificially in different substances, even in so homogeneous a body as wax poured in a melted state upon a surface of glass. Rocks in which the change has been superinduced are said to be cleaved, and the change itself is termed cleavage.

Considerable practice is required to be able to distinguish between the fissile structure thus developed by cleavage, and that due to original lamination of deposit. Should the rock consist of alternate bands of different textures, such alternation will of itself be sufficient to show the bedding; while a further test will be found in the frequent difference in the fineness of the cleavage as it passes from one rock into another. Fine-grained argillaceous rocks assume the most perfect cleavage; hence their value as slates. Sandy and gritty rocks do not allow of the development of such fine divisional planes. Consequently the cleavage lines may actually be seen to stop when they reach an arenaceous stratum and begin again on the further side at the next argillaceous band. Where no such intercalation of different strata can be observed, the geologist looks for lines of colour corresponding with original lamination. Should these fail, he may for an interval find it impossible to make sure in what direction the lines of stratification run. It will be perceived from Fig. 47 that cleavage runs independent of original bedding, coinciding with it or not, as the strata may happen to lie. The strike of the cleavage which can be traced with great persistence over