Page:Popular Science Monthly Volume 13.djvu/782

This page has been validated.

How Mountains are made.—In a paper by Prof. Joseph Le Conte, read before the National Academy of Sciences in April, and since published in the American Journal of Science, the formation of mountains is explained by the action of horizontal pressure resulting from interior contraction of the earth. The author considers all the principal types of mountain-structure, and appears to account for them very satisfactorily by this theory of horizontal pressure. It would be impossible, within the limits of a Miscellany article, to give an intelligible outline of the entire argument, and we must content ourselves with a synopsis of the author's remarks on the formation of mountains of a single anticlinal fold—the simplest conceivable mountain-structure. Here

PSM V13 D782 Ideal section of an anticlinal fold.png
Fig. 1.—Ideal Section of Simple Anticlinal Fold (unsysmmetrical).

the deeper strata are thickened and swelled upward by the horizontal pressure, while the upper strata are raised into a vault with little or no thickening, or may even be thinned and broken by tension. The vault is nearly always unsymmetrical, the yielding being greater on one side than on the other. In such cases a great fissure and slip is apt to occur on the steeper side, as shown in Fig. 2. Perhaps the best illustration of a

PSM V13 D782 Fissured and folded anticlinal fold.png
Fig. 2.—The same, with the strata Assured and faulted.

range of mountains of this simple type is the Uintah Mountains. A cross-section of this range shows a prodigious fault of 20,000 feet on the northern or steeper side of the original fold. If the crust of the uprising region be extremely rigid, the vault, instead of being forty or fifty miles across, as in the Uintah Mountains, may be one hundred or several hundred miles across; we have then a great plateau. And since an arch of such span, whether filled or not beneath with fused or semi-fused matter, cannot sustain itself, such elevated plateaus are peculiarly liable to fissure by breaking down of the arch, and to slips by gravitative adjustment of the broken parts. The result is conspicuous escarpments or conspicuous mountain ridges in the general direction of the axis of the plateau. Such, according to the author, is the origin of the north and south escarpments of the plateau-region of the Colorado, described by Powell, and of the north and south monoclinal ridges in the Basin-region, described by Gilbert and Howell. Again, a monoclinal fold may be modified by metamorphism. This, says the author, is especially apt to be the case if the strata be very thick and the fold narrow and high; that is, if the compression in a given space, and therefore the heat of compression, be very great. If, now, such a sharp fold, metamorphosed in its deeper strata along the line of greatest compression, be subjected to profound erosion, it forms a common type of mountain, viz., one consisting of a highly-metamorphic axis, flanked on either side by tilted strata corresponding to each other.


The Differences between Atlantic and Pacific Forests.—The differences between the Atlantic and Pacific forests of the United States are very striking in many respects. Prof. Asa Gray, in a recent lecture, presents a long list of Atlantic forest-trees that are either not at all or but feebly represented on the Pacific slope. For instance, the Pacific forest has no magnolias, no tulip-tree, no papaw, no linden or basswood, and is very poor in maples; no locust-trees, nor any leguminous tree; no cherry-tree large enough for a timber-tree; no gum-trees, nor sorrel-tree, nor kalmia; no persimmon; not a holly; only one ash that may be called a timber-tree; no catalpa, or sassafras; not a single elm nor hackberry; not a mulberry, nor planer-tree, nor maclura; not a hickory nor a beech, nor a true chestnut nor a hornbeam; barely one birch-tree, and that only far north, where the differences are less striking. As to coniferous trees, however, the only missing type is our bald cypress, the so-called cypress of our Southern swamps. "But as to our ordinary