Fig. 1.
We have, for instance, in Figure 1, a central granite mountain, with a succession of stratified beds sloping against its sides, while at its base are deposited a number of horizontal beds which have evidently never been disturbed from the position in which they were originally accumulated. The reader will at once perceive the method by which the geologist decides upon the age of such a mountain. He finds the strata upon its slopes in regular superposition, the uppermost belonging, we will suppose, to the Triassic period; at its base he finds undisturbed horizontal deposits, also in regular superposition, belonging to the Jurassic and Cretaceous periods. Therefore, he argues, this mountain must have been uplifted after the Triassic and all preceding deposits were formed, since it has broken its way through them, and forced them out of their natural position; and it must have been previous to the Jurassic and Cretaceous deposits, since they have been accumulated peacefully at its base, and have undergone no such perturbations.
The task of the geologist would be an easy one, if all the problems he has to deal with were as simple as the case I have presented here; but the most cursory glance at the intricacies of mountain-structure will show us how difficult it is to trace the connection between the phenomena. We must not form an idea of ancient mountain-upheavals from existing active volcanoes, although the causes which produced them were, in a modified and limited sense, the same. Our present volcanic mountains are only chimneys, or narrow tunnels, as it were, pierced in the thickness of the earth's surface, through which the molten lava pours out, flowing over the edges and down the sides and hardening upon the slopes, so as to form conical elevations. The mountain-ranges upheaved by ancient eruptions, on the contrary, are folds of the earth's surface, produced by the cooling of a comparatively thin crust upon a hot mass. The first effect of this cooling process would be to cause contractions; the next, to produce corresponding protrusions,—for, wherever such a shrinking and subsidence of the crust occurred, the consequent pressure upon the melted materials beneath must displace them and force them upward. While the crust continued so thin that these results could go on without very violent dislocations,—the materials within easily finding an outlet, if displaced, or merely lifting the surface without breaking through it,—the effect would be moderate elevations divided by corresponding depressions. We have seen this kind of action, during the earlier geological epochs, in the upheaval of the low hills in the United States, leading to the formation of the coal-basins.
On our return to the study of the American continent, we shall find in the Alleghany chain, occurring at a later period, between the Carboniferous and Triassic epochs, a good illustration of the same kind of phenomena, though the action of the Plutonic agents was then much more powerful, owing to the greater thickness of the crust and the consequent increase of resistance. The folds forced upward in this chain by the subsidence of the surface are higher than any preceding elevations; but they are nevertheless a succession of parallel folds divided by corresponding depressions, nor does it seem that the displacement of the materials within the crust was so violent as to fracture it extensively.
Even so late as the formation of the Jura mountains, between the Jurassic and Cretaceous periods, the character of the upheaval is the same, though there are more cracks at right angles with the general trend of the chain, and here and there the masses below have broken through. But the chain, as a whole consists of a succession of parallel folds, forming long domes or arches, divided by longitudinal valleys. The valleys represent the subsidences of the crust; the domes are the corresponding protrusions resulting from these subsidences. The lines of gentle undulation in this chain, so striking in contrast to the rugged and abrupt character of the Alps immediately opposite, are the result of this mode of formation.
After the crust of the earth had grown so thick, as it was, for instance, in the later Tertiary periods, when the Alps were uplifted, such an eruption could take place only by means of an immense force, and the extent of the fracture would be in proportion to the resistance opposed. It is hardly to be doubted, from the geological evidence already collected, that the whole mountain-range from Western Europe through the continent of Asia, including the Alps, the Caucasus, and the Himalayas, was raised at the same time. A convulsion that thus made a gigantic rent across two continents, giving egress to three such mountain-ranges, must have been accompanied by a thousand fractures and breaks in contrary directions. Such a pressure along so extensive a tract could not be equal everywhere; the various thicknesses of the crust, the greater or less flexibility of the deposits, the direction of the pressure, would give rise to an infinite variety in the results; accordingly, instead of the long, even arches, such as characterize the earlier upheavals of the Alleghanies and the Jura, there are violent dislocations of the surface, cracks, rents, and fissures in all directions, transverse to the general trend of the upheaval, as well as parallel with it.
Leaving aside for the moment the more baffling and intricate problems of the later mountain-formations, I will first endeavor to explain the simpler phenomena of the earlier upheavals.
Fig. 2.