Such is the doctrine of permanency of oceanic basins. It is undoubtedly a true doctrine, but must not be held in the rigid form characteristic of early thought. The forces originating oceanic basins still continue to deepen them and to increase the size and height of continents, but other forces are at work, some antagonizing (i. e., cutting down the continents and filling up the ocean beds), and still others determined by causes we little understand, by oscillations over wide areas, greatly modifying and often obscuring the effects of the basin-making movements. Here, then, we have two kinds of crust movements: the one fundamental and original, determining the greatest features of the earth and moving steadily onward in the same direction, ever increasing the features which it originates; the other apparently lawless, uncertain, oscillating over very wide areas, modifying and often obscuring the effects of the former. The old uniformitarians saw only the effects of the latter, because these are most conspicuous; the new evolutionists add also the former and show its more fundamental character, and thus introduce law and order into the previous chaos. The former is the one movement which runs ever in the same direction through all geologic time. The latter are the most common and conspicuous now and in all previous geologic time. The former underlies and conditions and unifies the history; the latter has practically determined all the details of the drama enacted here on the surface of the earth. Of the causes of the former we know something, though yet imperfectly. Of the causes of the latter we yet know absolutely nothing. We have not even begun to speculate profitably on the subject, and hence the apparent lawlessness of the phenomena. A fruitful theory of these must be left to the coming century.
Mountain Ranges.—If oceanic basins and continental domes constitute the greatest features of the earth’s face, and are determined by the most fundamental movements of the crust, surely next in importance come great mountain ranges. These are the glory of our earth, the culminating points of scenic beauty and grandeur. But they are so only because they are also the culminating points, the theaters of greatest activity, of all geological forces, both igneous and aqueous—igneous in their formation, and aqueous both in the preparatory sedimentation and in the final erosive sculpturing into forms of beauty. A theory of mountain ranges therefore lies at the bases of all theoretical geology. To the pre-geologic mind mountains are the type of permanence and stability. We still speak metaphorically of the everlasting hills. But the first lesson taught by geology is that nothing is permanent; everything is subject to continuous change by a process of evolution. Mountains are no exception. We know them in embryo in the womb of the ocean. We know the date of their birth; we trace their growth, their maturity, their decay, their death; we even find in the folded structure of the rock, as it were, the fossil bones of extinct mountains. In a word, we are able now to trace the whole life history of mountains.
Mountains, therefore, have always been a subject of deepest interest both to the popular and the scientific mind—an interest intensified by the splendors of mountain scenery and the perils of mountain exploration. The study of mountains is therefore coeval with the study of geology. As early as the beginning of the present century Constant Prevost observed that most characteristic structure of mountains—viz., their folded strata—and inferred their formation by lateral pressure. All subsequent writers have assumed lateral pressure as somehow concerned in the formation of mountains. But that the whole height of mountains is due wholly to this cause was not generally admitted or even imagined until recently. It was universally supposed that mountains were lifted by volcanic forces from beneath, that the lifted strata broke along the top of the arch, and melted matter was forced through between the parted strata, pushing them back and folding them on each side. And hence the typical form of mountain ranges is that of a granite axis along the crest and folded strata on each flank. But attention has lately been drawn to the fact that some mountains, as, for example, the Appalachian, the Uintah, etc., consist of folded strata alone, without any granite axis. In such ranges it is plain that the whole height is due not to any force acting from below, but to a lateral pressure crushing and folding the strata, and a corresponding thickening and bulging of the same along the line of crushing. Then the idea was applied to all mountain ranges. So soon as the prodigious amount of erosion suffered by mountains, greater often than all that is left of them, was fully appreciated, it became evident that the granite axis so characteristic of mountains was not necessarily pushed up from beneath and protruded through the parted strata, but was in many cases only a sub-mountain core of igneous matter slowly cooled into granite and exposed by subsequent erosion greatest along the crest.
Next, attention was drawn to the enormous thickness of the strata involved in the folded structure of mountains. From this it became evident that the places of mountains before they were formed were marginal sea bottoms off the coasts of continents, and receiving the whole washings of the continents. Thus the steps of the process of mountain formation were (1) accumulation of sediments on offshore sea bottoms until by pari passu subsidence an enormous thickness was attained. This is the preparation. (2) A yielding along these lines to the increasing lateral pressure with folding and bulging of the strata along the line of yielding, until the mountain emerges above the ocean and is added to the land as a coast range. This is mountain birth. (3) As soon as it appears above the water it is attacked by erosive agents. At first the rising by continuance of the crushing and bulging is in excess of the erosion, and the mountain grows. This is mountain youth. (4) Then supply and waste balance one another, and we have mountain maturity. (5) Then the erosive waste exceeds the growth by up-bulging, and mountain decay begins. (6) Finally, the erosive forces triumph and the mountain is clean swept away, leaving only the complexly folded rocks of enormous thickness to mark the place of a former mountain. This is mountain death. Such briefly is the life history of a mountain range.
In all this we have said nothing about causes. In this connection there are two points of especial importance: (1) Why does the yielding to lateral pressure take place along lines of thick sediments? (2) What is the cause of the lateral pressure?
1. Cause of Yielding to Lateral Pressure along Lines of Thick Sediments.—The earth was once very hot. It is still very hot within, and still very slowly cooling. If sediments accumulate upon a sea bottom the interior heat will tend to rise so as to keep at the same distance from the surface. If the sediments are very thick, say five to ten miles, their lower parts will be invaded by a temperature of not less than 500° to 1,000° F. This temperature, in the presence of water (the included water of the sediments), would be sufficient to produce softening or even fusion of the sediments and of the sea floor on which they rest. This would establish a line of weakness, and therefore a line of yielding, crushing, folding, bulging, and thus a mountain range. In the first formation of a range, therefore, there would necessarily be a sub-mountain mass of fused or semifused matter which by the lateral crushing might be squeezed into cracks or fissures, forming dikes. But in any case the sub-mountain mass would cool into a granite core which by erosion may be exposed along the crest. The explanation seems to be satisfactory.
2. Cause of the Lateral Pressure.—No question in geology has been more discussed than this, and yet none is more difficult and the solution of which is more uncertain. But the most obvious and as yet the most probable view is that it is the result of the secular contraction of the earth which has gone on throughout its whole history, and is still going on.
It is admitted by all that in an earth cooling from primal incandescence there must come a time when the surface, having become substantially cool and receiving heat also from the sun, would no longer cool or contract, but, the interior being still incandescently hot, would continue to cool and contract. The interior, therefore, cooling and contracting faster than the exterior crust, the latter following down the ever-shrinking nucleus, would be thrust upon itself by a lateral or tangential pressure which would be simply irresistible. If the earth crust were a hundred times more rigid than it is, it still must yield to the enormous pressure. It does yield along its weakest lines with crushing, folding, bulging, and the formation of mountain ranges.
This is the barest outline of the so-called “contractional theory of mountain formation.” Very many objections have been brought against it, some of them answerable and completely answered, but the complete answer to others must be left to the next century. Perhaps the greatest objection of all is the apparent insufficiency of the cause to produce the enormous amount of folding found not only in existing mountains but in the folded structure of rocks where mountains no longer exist. But it will be observed that I have thus far spoken only of contraction by loss of heat. Now, not only has this cause been greatly underestimated by objectors, but, as shown by Davison and especially by Van Hise, there are many other and even greater causes of contraction. It would be out of place to follow the discussion here. The subject is very complex, and not yet completely settled.
We have given the barest outline of the history of mountain ranges and of the theory of their formation as worked out in the last third of the present century, and, I might add, chiefly by American geologists. So true is this, that by some it has been called the “American theory.”