The manner in which mountain ranges have been formed must be deduced from a careful study of their structure, for no one has ever witnessed the process of that formation. Mountain building may be going on at the present time; indeed, there is no reason to suppose that it is not, but so slowly is the work carried on that it withdraws itself entirely from observation. Nevertheless, the general course of events may be inferred with much confidence from the structure of the range.

The first step in the formation of a mountain range must evidently be the accumulation of an immensely thick body of strata. This, of course, must have taken place chiefly under water, and the only body of water large enough is the sea. Furthermore, our studies of modern marine deposits have taught us that thick strata can be accumulated only in rather shallow water and parallel with shore-lines. This shoal-water origin of their strata is confirmed by the examination of actual mountain ranges, where we find great masses of conglomerates, ripple-marked and sun-cracked sandstones and shales, and abundant other testimony of deposition in shallow water, in deltas and on flood plains of rivers. To accumulate thick strata in shoal water, the bottom must subside as the sediments are piled upon it, else the water would be filled up and deposition cease. Such a sinking trough is a geosyncline, and in geosyclines filled with sediments is the cradle of the mountains. The area of the trough varies from time to time, as do also the position of the line of maximum subsidence and the relative rate of depression and sedimentation, so that the depth of water varies.

We saw above that the strata of .mountain ranges are very much thicker than the same strata in the adjoining plains, which means that the ranges have been formed along the lines of maximum sedimentation.

The second stage in the building of a range is the upheaval of the thick mass of strata into a series of anticlinal and synclinal folds, which may be upright, open, and symmetrical, or closed, asymmetrical, inclined, or inverted. This, as we have already learned, can be produced only by lateral compression, a conclusion which is sustained not only by the mechanics of folding and faulting, but also by the less obvious structures, such as cleavage and fissility, metamorphism, the microscopic crumplings and plications, and the crushing and flowage of the mineral particles. The compressing force does not raise anticlines with great cavities beneath them, for such arches could not well be self-supporting, but mashes together the whole mass of strata, raising them into folds and wrinkles, crowding the beds into a greatly reduced breadth; or, when they are not sufficiently loaded to be plastic, breaking and dislocating them in great thrusts. It is not necessary to suppose that a mountain range was thrown up by one steady movement.

On the contrary, there is good reason to believe that repeated movements, separated, it may be, by long intervals of time, have been engaged in the work.

The great forces of compression which have upheaved mountain ranges have manifested themselves recurrently from the earliest to the latest recorded periods of the earth's history, and from these recurrences form conspicuous landmarks in the chronological scheme.

There are certain mountain ranges which have a different structure and must have had a correspondingly different mode of origin. As already pointed out, in the Great Basin, which lies between the Sierra Nevada and the Wasatch Mountains, are a number of parallel mountain ranges with a prevalent north and south trend, which are collectively called the Basin Ranges. These mountains are not folds of very thick strata, but tilted fault-blocks, which have been made by normal faults, each upthrow side standing as a great escarpment, but with a tilted top that gradually slopes back to the foot of the next block, to which it stands as the downthrow side. The processes of denudation have carved these tilted blocks into peaks and ridges of the ordinary kind. The boundary ranges, the Sierra Nevada and the Wasatch, although mountains of folding, have themselves been modified by the same process, for each of these ranges has a great fault along its base, the Great Basin being on the downthrow side with reference to each of them.

The Date of Mountain Ranges means the geological period in which they were first upheaved above the sea. This date is subsequent to the newest strata which are involved in the movement, and earlier than that of the oldest strata which did not take part in it, but must have done so, had they been present. Strata which rest unconformably against the flanks of a range must have been deposited after the folding movement was accomplished. If the newest folded strata and the oldest unmoved strata be of successive geological periods, the date of the upheaval is placed between those two periods and said to close the older one for the particular region involved. The subsequent history of a mountain range after its final upheaval above the sea must be read in its denudation and in the evolution of its topography and drainage.