ains is the flexing and folding to which reference has already been made. This structure varies in complexity. Occasionally, as in the case of the Uinta Mountains, the folding is of the simplest kind, the strata being arched up in a single, broad saddleback or anticline (Fig. 2). Or the rocks may be arranged in a series of open,

Fig. 2. Geological Structure of the Uinta Mountains. A symmetrical anticline, dislocated by a normal fault at f. The dotted line suggests the former extension of the strata.

symmetrical waves or undulations, as in the ranges of the Western Jura and the sandstone mountains of the Carpathians (Fig. 3).

Fig. 3. Symmetrical Folds of the Carpathians. Compare with Fig. 1 in which the form of the ground coincides more or less closely with the geological structure.

More usually, however, the structure is infinitely complex,-the strata showing closely compressed, unsymmetrical folds, (Fig. 4)

Fig. 4. Diagrammatic Section of Unsymmetrical Folds.

inclined at all angles or even overturned and lying on their sides, while ever and anon the structure is still further confused by great fractures and horizontal displacements, and not infrequently by the intrusion of numerous smaller and larger masses of igneous rock. No drawing or even photograph can give an adequate idea of this remarkable architecture. Not only do we see large folds, some of which may measure many hundreds of feet from crest to base, but within each compressed fold we can recognize innumerable subordinate flexures, contortions, and crumplings, varying in importance down to puckerings so minute as to be visible only under the microscope.

Another phenomenon constantly encountered in regions of highly folded rocks is what is known as slaty cleavage. In this structure the rocks are traversed by close, parallel division-planes, which coincide in direction with the axes of the folds (Fig. 5).

Fig. 5. Slaty Cleavage. The undulating lines represent folded strata ; the close-set, steeply inclined lines represent slaty cleavage.

When cleavage is well developed the rocks become coarsely or finely fissile, according as they are composed of coarser or finer ingredients. In many cases, moreover, the surfaces of the cleavageplanes, as they are termed, acquire a silvery glaze or lustre, due to the development of mica. As the complexity of the rock arrangements increases, however, cleavage-planes begin to disappear and are by and by replaced by the structure known as foliation or schistosity. Schistose or foliated rocks are more or less distinctly crystalline in texture, the component minerals being arranged in rudely alternate lenticular layers or laminæ. Now it

is specially worthy of note that rocks of this kind predominate in those parts of a mountain-chain where folding and contortion and other evidence of deformation are most conspicuous.

The facts thus briefly recapitulated have convinced observers that folded mountains owe their origin to pronounced lateral compression. The folds themselves are evidence sufficient in itself to prove the case, but when we enter into greater detail we find assurance made doubly sure. Examined under a microscope the mineral constituents of a fine-grained, well cleaved rock, like common roofing-slate, are seen to be flattened out and extended in the direction of the cleavage-planes, and these planes, it will be remembered, have the same trend as the axes or axial planes of the large rock folds. Obviously the folds have been strongly compressed from side to side, and the slaty structure has been superinduced as a result of this squeezing process. Even the unaided eye may oftentimes detect conspicuous evidence of crushing and squeezing. The stones of a conglomerate, for example, may be flattened and drawn out as if by compression acting in a direction at right angles to the trend of the great anticlines and synclines; and the same is the case with fossils which in highly folded strata are frequently so distorted that the geologist may have great difficulty in recognizing their species.

Further, a close study of the crystalline rocks, which so frequently enter into the formation of a complex mountain region, shows that these have sometimes been subjected to such enormous pressure that their constituent minerals have been crushed, flattened, and sheared. Whole rock masses, in short, have been compelled to flow as if they were plastic bodies. So great has been the force applied that complex chemical and mineralogical changes have been induced, even fragmental rocks of aqueous origin having become crystalline and schistose. In a word, metamorphism, more or less pronounced, is a frequent concomitant of rock folding on a large scale, and especially affects those rock masses which have been most severely pressed and mashed together in a mountain-range.

To sum up we note that in the less closely folded parts of a mountain-range the original character of the component rocks may still be recognized, they may be cleaved and rendered in this way more or less fissile, but these changes are not so pronounced as to prevent us telling what the rocks originally were. As we approach the region of highly complicated structures, however, rock changes become more and more marked, until frequently crystallization and foliation combine to destroy all original rock characters. Aqueous rocks of various kinds are metamorphozed into schists, while massive igneous rocks, such as granite, have likewise been crushed and foliated; nay, even ancient crystalline schistose formations have been reconstituted,-new planes of foliation obscuring and not infrequently entirely obliterating the older structures.

Having now outlined the evidence which leads to the belief that the structure of folded mountains is the result of lateral

Fig. 6. Recumbent Fold passing into a Thrust-fault. f,f, Thrust-fault.

Fig. 8. Strata Dislocated and Displaced Without Preliminary Folding. t, t, t, Thrust-fault.

compression, we may next consider certain general structural features of a typical mountain-range. It is obvious that detailed investigation of the folds ought to throw some light on their origin,-we should be able from such investigation to ascertain the point from which came the lateral thrust that produced them. So far as our present knowledge goes, it would seem that in most mountainchains the majority of the unsymmetrical folds lean over in one and the same direction, and it is inferred, therefore, that this indicates the trend of crustal movement. If the crests of most of the anticlines look towards the north, for example, then the movement must have been northerly. Inferences of this kind are strongly supported by the fact that the great horizontal or approximately horizontal displacements in a mountain-range have the same trend as the folds. If the latter lean over to the north the displaced rock masses are found to have moved towards the same point. Horizontal displacements of the kind referred to have often played a conspicuous rôle in the process of mountain making. The rocks have yielded to thrust not only by folding, but by rupturing. In the case of recumbent folds the crests of the closely compressed anticlines have often given way, and the one limb of a fold has been thrust for