where the rocks are more or less steeply inclined and contorted. Normal faults, on the other hand, occur under all conditions of rock-structure-traversing alike horizontally arranged strata and inclined and folded beds of every kind.

So much, then, for the general types of structure met with among highly folded strata. So far as our present knowledge goes, complex folding, such as is

seen in true mountains of uplift, has resulted from horizontal movement in one direction. This is shown by the manner in which most of the more closely compressed and steeper folds of a mountain-chain. tend to lean over one way. Under the influence of an irresistible horizontal thrust the strata find relief by folding, and the crust bulges upwards, the flexured rocks naturally bending over in the direction of least resistance. The resulting structure may be shown diagrammatically as in Fig. 37. In this diagram only

folds are represented; in many cases, however, the rocks are not merely flexed, folded, and contorted, but dislocated and displaced. Frequently, indeed, they have yielded to the intense pressure by shearing, and slice after slice, hundreds or even thousands of feet in thickness, has been pushed forward and piled one on top of the other. Although the closer folds tend as a rule to lean over in the direction of crustal movement, yet occasionally they are inclined in the opposite direction, thus giving rise to the well-known

FIG 37. DIAGRAM OF MOUNTAIN FLEXURes.

The arrow shows the direction of thrust.

fan-structure seen in the anticlinal double-fold, Fig. 36. Now and again, too, the folds may open out, and so form symmetrical flexures with vertical axes, or normal anticlines and synclines. The cause of such variations in the folding of the strata is an interesting question, but does not concern us here.

When a tract of highly disturbed rocks has been exposed to erosion for a very prolonged period, it is usually hopeless to attempt to reconstruct the original configuration of the ground, save in a very general way. The primeval land-forms that may have resulted from crustal deformation have been entirely remodelled or removed by denudation. But there

are many regions where similar extensive deformation has taken place at a relatively recent geological date, and where, therefore, time has not sufficed for the obliteration of all surface-features due to crustal disturbance. In the younger mountain-chains of the world, underground structure and orographical features to a certain extent coincide. The study of these mountains, therefore, enables us to realise the conditions that formerly obtained in tracts of highly complicated structure, from which, under long-continued erosion, all trace of the original configuration of the ground has vanished. Not only so, but the havoc wrought by epigene action upon even the youngest of our mountains shows us how and by what means the complicated mountain-chains of earlier days have gradually been reduced. For, just as lands built up of horizontal and gently inclined strata have experienced various degrees of erosion, thus enabling us to trace the successive stages through which such lands must pass, so regions of highly complex structure present us with various phases of denudation. And thus, by comparing one tract with another, we may spell out the whole story; and in the degraded relics of former mountain-systems we read the fate that must eventually overtake the proudest elevations of the present.

The study of the land-forms assumed by highly flexured strata should naturally begin with the examination of some young mountain-chain. But even the youngest of such mountains has already under

gone much erosion, and its structure is often extremely complicated. To examine any one system in detail, and to follow the whole process of its denudation, would be a laborious work, far beyond the limits of our present inquiry. All that we desire is to ascer tain if we can how far geological structure and orographical configuration coincide during the period of a mountain's infancy and early youth, and by what means its original form becomes modified and eventually remodelled. For this purpose we may profitably begin our study by considering first some hypothetical case. We shall suppose, then, that under tangential pressure the horizontal strata of some region have bulged up and become folded along a given line or Under such conditions great faults and thrustplanes would be likely enough to occur; but for the sake of simplicity we shall ignore these, and fix our attention only on the flexing and folding. We shall suppose further that our mountain-chain is the result of one prolonged continuous earth-movement. How, then, will the elevation of the strata affect the surface? Will the complex folding of the rocks give rise to similar intricate deformations of the surface? This does not necessarily follow, for, were the movement of elevation very slow and protracted, the gradually rising surface might be so continually reduced by denudation that underground structure and external form would rarely or never correspond. But, on the other hand, were the rate of elevation in excess of the rate of erosion, the larger folds of the strata

might be expected to give rise to similar undulations at the surface. It is very doubtful, however, whether the latter would ever be as strongly pronounced as the former; for at great depths the folds would be pressed closely together, while they would naturally tend to open out upwards into broader undulations. Hence, deeply buried rock-masses might be intensely flexed and folded, while the surface might show only a more or less pronounced bulging. The infant mountain might appear as merely one single long swell or undulation, with smooth slopes, declining at no great angle to the low grounds. Or there might be a series of two or more such undulations. The study of existing mountain-chains, however, leads to the belief that in some cases at least very considerable deformation of the surface has accompanied mountain-making, all the larger folds of the strata being probably at first represented above ground by corresponding ridges and depressions.

We do not know whether the elevation of a mountain-chain was ever suddenly effected. So far as we can judge from the evidence supplied by geological structure, it would seem as if the horizontal movements of the crust had been gradual and protracted, and often interrupted by long pauses. There is little reason to doubt, however, that during the growth of a mountain-chain sudden snapping of rocks under pressure must have occurred frequently enough, and that earthquakes of greater or less intensity must have accompanied the upheaval. If such has been