Page images

cut up into segments, and these progressively diminish in extent as erosion proceeds. Every table-land, in short, tends to acquire an irregular mountainous aspect. As examples of highly eroded plateaux of accumulation may be cited the Plateau of the Colorado, the Uplands of Abyssinia, and the Deccan of India. Plateaux of erosion, as might have been expected, are far more common, many excellent examples occurring in our own continent, such as the highly denuded plateaux of Scandinavia and Scotland and the plateau of Central France.

3. Hills and Mountains. Just as we cannot separate plains from plateaux by any hard-and-fast line, so we find it impossible to distinguish clearly between hills and mountains. In general we may say that the term hill is properly restricted to more or less abrupt elevations of less than 1000 ft., all the altitudes exceeding this being mountains. The terms, however, are loosely used, for in very lofty mountain regions eminences considerably above 1000 ft. are spoken of as hills, while in low-lying tracts heights of only a few hundred feet not infrequently become dignified with the name of mountains. It is obvious, in short, that just as plains merge into plateaux, so there must be a gradual transition from hills into mountains. For purposes of classification, therefore, it is not necessary to distinguish between the latter, and we shall treat of them both under the common head of mountains. From our present point of view, then, a mountain is simply a more or less abrupt elevation, or somewhat sudden increase in the slope of a landsurface, and may be of any height from less than one hundred feet upwards. It may also be of any extent, and either isolated or more or less closely associated with other elevations, forming regular or irregular groups or definite ranges. Notwithstanding the great differences of elevation, of form, and of arrangement of hills and mountains, it is obvious that all these fall naturally into two divisions, namely, (a) elevations which have been formed as such either by epigene or by hypogene action, and (b) elevations which have been carved out of pre-existing rockmasses by epigene action alone. To avoid periphrasis, we shall speak of these two kinds of elevations as original or tectonic mountains, and subsequent or relict mountains, respectively.

(a) Original or Tectonic Mountains. Under this head come many of the most insignificant as well as the majority of the greater elevations of the globe. Some of these have been piled or heaped up at the surface—they have grown into heights by gradual accumulation, and may therefore be termed accumulation-mountains. This group naturally includes all volcanic cones and hills, geyser mounds, mud-volcanoes, etc. Many of these, no doubt, are mere monticles and hillocks, but all alike owe their origin to the extrusion of materials from below and the accumulation of these at the surface. Of much less importance are the eminences formed by the direct action of epigene agents, hardly any of which ever reach the height and dimensions that are usually associated with the term mountain. Nevertheless, they form not infrequently conspicuous land-features, and cannot be ignored in our classification of land-forms. Among them are included morainic and fluvio-glacial hills and ridges of every kind, sand-dunes, etc.

But by far the most important tectonic mountains are those which have resulted from the flexuring and fracturing of the earth■s crust,—deformation-mountains, as they may be termed. All the great mountain-ranges of the globe come under this group. The majority of these owe their origin essentially to tangential pushing and crushing; they consist for the most part of flexed and contorted rocks. Now and again, however, we meet with mountain-ranges the rocks of which may show no conspicuous folds and flexures. Ranges of this kind have been determined by series of great parallel fractures and dislocations of the crust; the ranges are, in short, vast rectangular blocks of strata which may not otherwise be much disturbed. The Alps, the Himalayas, and the Cordilleras of America are typical examples of deformation-mountains composed of highly folded rocks. Dislocations are, of course common enough among such chains and ranges, but their distinguishing character is the folding and contortion—hence they are termed folded or flexured mountains. The faulted ranges of the Great Basin (North America) are notable examples of the other kind of deformation-mountains. In these ranges the strata are sometimes horizontal, or approximately so, but are more usually inclined. Folding and flexing may be absent, or only partially and locally developed. The characteristic features of such ranges are the great faults that bound them, and hence they may be spoken of as dislocation-mountains. In the same category would come the Horste of German geologists. These are mountains bounded by dislocations—they project above the general level because the rocks surrounding them have been dropped down by faulting. Under the head of deformation-mountains we may also include those gibbosities, or prominent swellings of the surface, caused by the intrusion below of masses of molten matter. They are typically represented by the Henry Mountains of Utah and the Elk Mountains of Colorado, and may be termed laccolith-mountains.

Of course, all deformation-mountains are more or less denuded, some of them to such an extent that their original configuration can only be guessed at. But since they owe their elevation above adjacent lowlands to crustal movements, they are entitled to be classed as tectonic mountains.

(b) Subsequent or Relict Mountains. Mountains belonging to this great class frequently form irregular groups,—there is often an absence of arrangement in separate parallel or interosculating ridges and ranges such as characterises tectonic mountains. This absence of alignment or orientation, however, is by no means general, and is most characteristic of relict

[ocr errors]

mountains which have been carved out of horizontal and gently undulating strata, the strike of which is constantly changing. When the strike runs persistently for long distances in one direction, the mountains in such a region now and again form more or less parallel ranges, having the same trend as the strike.

The direction and to a large extent the shape or form of relict mountains are thus mainly determined by the geological structure. They are the more salient portions of plateaux which are in process of being reduced to some base-level of erosion. Plateaux of accumulation are eventually cut up into segments, which, progressively diminishing in extent and height, form irregular groups of tabular and pyramidal hills and mountains. Hills and mountains hewn out of plateaux of erosion, on the other hand, not infrequently simulate the arrangements that are most characteristic of deformation-mountains. Should the strata consist of a thick series of relatively soft rocks, with here and there interbedded rocks of a less yielding kind, all dipping at a moderate angle in one direction, the outcrops of the harder rocks eventually come to project prominently. We thus have long lines or ranges of escarpments, separated from each other by parallel hollows. When the strata dip at a high angle, however, the outcrops of the harder rocks often form series of narrower and broader ridges, rather than well-marked escarpments and dip-slopes, but the ridges continue to be separated by strike-valleys. Even when the rocks of a plateau are highly contorted and

« PreviousContinue »