Common Minerals and Rocks

by William O. Crosby

Fig. 37.—Columnar dike.

2. Contraction joints or shrinkage cracks.—That many cracks in rocks are due to shrinkage, there can be no doubt. The shrinkage may result from the drying of sedimentary rocks; but more generally from the cooling of eruptive rocks. Every one has noticed in warm weather, the cracks in layers of mud or clay on the shore, or where pools of water have dried up; and we have already seen that these sun-cracks are often preserved in the hard rocks. They have certain characteristic features by which they may be distinguished from the joints of the first class. They divide the clay into irregular, angular blocks, which often show a tendency to be hexagonal instead of quadrangular. The cracks are continually uniting and dividing, but are not parallel, and rarely cross each other. Sun-cracks never affect more than a few feet in thickness of clay, and are an insignificant structural feature of sedimentary rocks. In eruptive rocks, on the other hand, the contraction joints have a very extensive, and, in some cases, a very perfect development, culminating in the prismatic or columnar jointing of the basaltic rocks. This remarkable structure has long excited the interest of geologists, and, although the basalt columns were once regarded as crystals, and later as a species of concretionary structure, it is now generally recognized as the normal result of slow cooling in a homogeneous, brittle mass. The columns are normally hexagonal, and perpendicular to the cooling surface, being vertical in horizontal sheets and lava flows, as in the classic examples of the Giant’s Causeway and Fingal’s Cave, and horizontal in vertical dikes ([Fig. 37]). They begin to grow on the cooling surface of the mass and gradually extend toward the centre, so that dikes frequently show two independent sets of columns.

3. The concentric joints of granitic rocks.—In quarries of granite and other massive crystalline rocks, it is often very noticeable that the rock is divided into more or less regular layers by cracks which are approximately parallel with the surface of the ground, some of the granite hills having thus a structure resembling that of an onion. The layers are thin near the surface, become thicker and less distinct downwards, and cannot usually be traced below a depth of fifty or sixty feet. These concentric cracks are of great assistance in quarrying, and are now regarded as due to the expansion of the superficial portions of the granite caused by the heat of the sun. In reference to this view of their origin these may be properly called expansion joints.

Structure of Mountain-chains.—Mountains are primarily of two kinds,—volcanic and non-volcanic. The structure of the former belongs properly with the original structures of the volcanic rocks; but the latter—the true mountains—owe their internal structure and altitude or relief almost wholly to the crumpling and mashing together of great zones of the earth’s crust, being, as already pointed out, the culminating points of the plication, cleavage, and faulting of the strata. “A mountain-chain consists of a great plateau or bulge of the earth’s surface, often hundreds of miles wide and thousands of miles long. This is usually more or less distinctly divided by great longitudinal valleys into parallel ranges and ridges; and these, again, are serrated along their crests, or divided into peaks by transverse valleys. In many cases this ideal chain is far from realized, but we have instead, a great bulging of the earth’s crust composed on the surface of an inextricable tangle of ridges and valleys of erosion, running in all directions. In all cases, however, the erosion has been immense; for the mountain-chains are the great theatres of erosion as well as of igneous action. As a general fact, all that we see, when we stand on a mountain-chain—every peak and valley, every ridge and cañon, all that constitutes scenery—is wholly due to erosion.”—Le Conte.

The structure of mountains thus fells under two heads: (1) The internal structure and altitude, which are due to the action of the subterranean agencies. (2) The external forms, the actual relief, which are the product chiefly of the superficial agencies or erosion. The study of mountains has shown that: (1) They are composed of very thick sedimentary formations. Thus the sedimentary rocks have a thickness of 40,000 feet in the Alleghanies; of 50,000 feet in the Alps; and of two to ten miles in all important mountain-chains. Such thick deposits of sediments, as we have already seen, must be formed on a subsiding sea-floor, and in many mountain-chains, as in the Alleghanies, the great bulk of these sediments are still below the level of the sea. Again, thick sedimentary deposits can only be formed in the shallow, marginal portions of the sea; and when such a belt of thick shore deposits yields to the powerful horizontal thrust, and is crumpled and mashed up, it is greatly shortened in the direction of the pressure and thickened vertically, so that its upper surface is lifted high above the level of the sea, and a mountain-chain is formed and added to the edge of the continent. We thus find an explanation of the important fact that on the several continents, but notably on the two Americas, the principal mountain-ranges are near to and parallel with the coast lines.

2. The mountain-forming sediments are usually strongly folded and faulted, and exhibit slaty cleavage wherever they are susceptible of that structure; and the older rocks, especially, in mountains are often highly metamorphosed, and are traversed by numerous veins and dikes, the infallible signs of intense igneous activity.

“In other words, mountain regions have been the great theatres—(1) of sedimentation before the mountains were formed; (2) of plication and upheaval in the formation of the range; and (3) of erosion which determined the present outline. Add to these the metamorphism, the faults, veins, dikes, and volcanic outbursts, and it is seen that all geological agencies concentrate there.”—Le Conte.

Since mountain-ranges are great up-swellings or bulgings of the strata, their structure is always essentially anticlinal; and they sometimes consist of a single more or less denuded anticline ([Fig. 38]), the oldest and lowest strata exposed forming the summit of the range. More commonly, however, the single great arch or uplift is modified by a series of longitudinal folds, as shown in the section of the Jura Mountains ([Fig. 21]). Still more commonly the folds are closely pressed together, overturned, broken, and almost inextricably complicated by smaller folds, contortions, and slips.

Fig. 38.—Anticlinal mountain.