If these segments be regarded as the great integers of body-movement, two-thirds of them taking precedence in sinking and the other third in suffering distortion, it is easy to pass to the conception of sub-segments, moving somewhat differently from the main segments, so as to aid in their adjustment to one another, and thus to the conception of plateaus and deeps. It is easy also to pass to the conception of mutual crowding and crumpling at the edges of these segments, accompanied by fracture and slipping. These conceptions perhaps represent the true relations between the massive movements of the abysmal and continental segments, as well as the less massive plateau-forming movements and the mountain-forming distortions. The mountains and plateaus are probably the incidental results of the great abysmal and continental readjustments.
The great movements are probably to be attributed to stresses that gradually accumulated until they overcame the rigidity of the thick massive segments involved, and forced a readjustment. In accumulating these stresses, some local yielding on weak lines and at special points was an inevitable incident in distributing more equably the accumulating stresses. So, also, the first great readjustments probably left many local strains and unequal stresses which gradually eased themselves by warpings, minor faultings, etc., so that some minor movements were a natural sequence of the great movements. But there were doubtless many local and superficial causes, such as irregular gains and losses of heat, regional loading and unloading, solution, hydration, etc., that have caused local or regional movement, and which have little to do with the great deformations of the earth’s body. As implied above, the gentle, nearly constant movements probably fall mainly into a different category from the great periodic movements. Both will be considered further.
The differential extent of the movements.—Between the highest elevation of the land and the lowest depth of the ocean, there is a vertical range of nearly twelve miles. There may have been higher elevations, relatively, in past times, but probably not deeper depressions; and so, if we assume that the surface was once perfectly spheroidal, this may be taken as a maximum expression of differential movement, not absolute vertical movement. From the Thibetan plateau, where a considerable area exceeds three miles in height, to the Tuscarora deep, where a notable tract exceeds five miles in depth, the range is eight miles, which may fairly represent the vertical range of rather massive differential movement. From the average height of the continents to the average abysmal bottoms of the oceans the range is nearly three miles, which may be taken as the differential movement of the great segments. Under certain hypotheses of the origin and early history of the earth, to be sketched later, the surface is not assumed to have been perfectly spheroidal originally, and hence the present irregularities do not necessarily imply so great differential movement.
If the protruding portions of the lithosphere were graded down and the basins graded up to a common level, this level would lie about 9000 feet below the ocean-surface. This equated level is the best basis of reference for relative segmental movements. Referred to this datum plane, the continents, having an area about half as great as that of the ocean depths, have been squeezed up relatively about two miles, and the basins have sunk about one mile from the ideal common plane. The total downward movement, representing the total shrinkage of the earth, is quite unknown from observation. It is probably very much greater than the differential movement, as will appear from theoretical considerations as we go on.
The extent of the lateral movements has a peculiar interest, for it bears theoretically on the shrinkage of the earth. Every mile of descent of the crust represents 6 miles (6.28) shortening of the circumference. If the vertical movements were limited to the relative ones just named, the mile of basin descent would give but little more than 6 miles of surplus circumference for lateral thrust and crumpling. How far does this go in explaining the known facts? By measuring the folds of the Alps, Heim has estimated the shortening represented by them to be 74 miles.[243] Claypole estimated the shortening for the Appalachians in Pennsylvania, not including the crystalline belt on the east, at 46 miles;[244] McConnel placed that of the Laramide range in British America at 25 miles,[245] and LeConte that of the Coast range in California at 9 to 12 miles.[246] These estimates must be corrected for the thickening and thinning of the beds in the process of folding, for the composite character of the folds, and for the effects of shearing and faulting. These will in part tend to increase and in part to decrease the estimates. The first effect of horizontal thrust is to close up all crevices and compact the beds as much as they will stand without bending. A part of the unusual thickness which the beds of folded regions commonly show is probably due to this edgewise compression. In experiments on artificial strata made to illustrate foldings ([Fig. 449a]), the thickening of the layers is a very appreciable part of the process, though probably natural beds do not thicken in equal proportion. After the beds have been closely folded and the thrust is athwart them, they are thinned and stretched on the limbs of the fold. How far this and other causes of extension offset initial compression is undetermined, and is differently estimated. It seems highly probable from the nature of the case that the edgewise compression which resulted from sustaining the full stress before the beds bent, was much greater than the crosswise compression on the limbs of the folds, which came into action only after the stress had been largely satisfied by folding.
Fig. 449a.—Illustrations of Willis’ experiments in the artificial representation of mountain folding. The sections were formed of layers of wax of different colors, and were mechanically compressed from the right. The upper section shows the original state, and the offsets of the succeeding sections at the right indicate the amount of shortening. (Thirteenth Ann. Rep. U. S. Geol. Surv.)
Whatever the correction, and whatever the probable errors of the above estimates, the amount of shortening involved in folding is large. The estimates given are merely those for certain periods of folding, and represent only that portion of the compression of the circumference which was concentrated in a given mountain range. The whole shortening of a circumference is to be found by adding together all the transverse foldings on a given great circle, following it about the globe at right angles to a given folded tract. In so doing, it will be seen that the belt does not usually cross more than one or two strongly folded tracts of the same age, from which it is inferred that the shortening on each great circle was largely concentrated in a few tracts running at large angles to each other, to accommodate the shrinkage of the globe in all directions. If the folding in a main range crossing any great circle is doubled, it will probably represent roughly the shortening for that entire circle for that age. If one is disposed to minimize the amount of folding, the estimate may perhaps be put roundly at 50 miles, on an entire circumference, for each of the great mountain-making periods. If, on the other hand, one is disposed to give the estimates a generous figure so as to put explanations to the severest test, he may perhaps fairly place the shortening at 100 miles, or even more. For the whole shortening since Cambrian times, perhaps twice these amounts might suffice, for while there have been several mountain-making periods, only three are perhaps entitled to be put in the first order, that at the close of the Paleozoic, that at the close of the Mesozoic, and that in the late Tertiary. The shortening in the Proterozoic period was considerable, but is imperfectly known. The Archean rocks suffered great compression in their own times, and probably shared in that of all later periods, and if their shortening could be estimated closely, it might be taken as covering the whole. Assuming the circumferential shortening to have been 50 miles during a given great mountain-folding period, the appropriate radial shrinkage is 8 miles. For the more generous estimate of 100 miles, it is 16 miles. If these estimates be doubled for the whole of the Paleozoic and later eras, the radial shortening becomes 16 and 32 miles, respectively.