Common Minerals and Rocks

by William O. Crosby

An impregnation is an irregular segregation of metalliferous minerals in the mass of some eruptive or crystalline rock. Its outlines are not sharply defined, but it shades off gradually into the enclosing rock.

Fahlbands are similar ill-defined deposits or segregations in stratified rocks. An impregnation or vein occurring along the contact between two dissimilar rocks is called a contact deposit. These are usually found between formations of different geological ages, and especially between eruptive and sedimentary rocks.

Subsequent Structures produced by Subterranean Agencies.

The subterranean forces concerned in the formation of rocks are chiefly various manifestations of that enormous tangential pressure developed in the earth’s crust, partly by the cooling and shrinkage of its interior, but largely, it is probable, by the diminution of the velocity of the earth’s rotation by tidal friction, and the consequent diminution of the oblateness of its form. It is well known that the centrifugal force arising from the earth’s rotation is sufficient to change the otherwise spherical form of the earth to an oblate spheroid, with a difference of twenty-six miles between the equatorial and polar diameters. It is also well known that while the earth turns from west to east on its axis, the tidal wave moves around the globe from east to west, thus acting like a powerful friction-brake to stop the earth’s rotation. Our day is consequently lengthening, and the earth’s form as gradually approaching the perfect sphere. This means a very decided shortening and consequent crumpling of the equatorial circumference, and is equivalent to a marked shrinkage of the earth’s interior, so far as the equatorial regions are concerned.

The most important and direct result of the horizontal thrust, whether due to cooling or tidal friction, is the corrugation or wrinkling of the crust; and the earth-wrinkles are of three orders of magnitude,—continents, mountain-ranges, and rock-folds or arches.

Continents and ocean-basins, although the most important and permanent structural features of the earth’s crust, do not demand further consideration here, since their forms and relations are adequately described in the better text-books of physical geography. The forms and distribution of mountain-ranges might be dismissed in the same way; but, unlike continents, the structure of mountains, upon which their reliefs mainly depend, is quite fully exposed to our observation, and is one of the most important fields of the student of structural geology. Mountains, however, as previously explained, combine nearly all the kinds of structure produced by the subterranean agencies, and their consideration, therefore, belongs at the end rather than the beginning of this section.

Inclined or Folded Strata.—Normally, strata are horizontal, and dikes and veins are vertical or nearly so. Hence the stratified rocks are more exposed to the crumpling action of the tangential pressure in the earth’s crust than the eruptive and vein rocks; and it is for this reason and partly because the stratified rocks are vastly more abundant than the other kinds, that the effects of the corrugation of the crust are studied chiefly in the former. But it should be understood that folded dikes and veins are not uncommon.

That the stratified rocks have, in many instances, suffered great disturbance subsequent to their deposition, is very evident; for, while the strata must have been originally approximately straight and horizontal, they are now often curved, or sharply bent and contorted, and highly inclined or even vertical. All inclined beds or strata are portions of great folds or arches. Thus we may feel sure when we see a stratum sloping downward into the ground, that its inclination or dip does not continue at the same angle, but that at some moderate depth it gradually changes and the bed rises to the surface again. Similarly, if we look in the opposite direction and think of the bed as sloping upward—we know that the surface of the ground is being constantly lowered by erosion, and consequently that the inclined stratum formerly extended higher than it does now, but not indefinitely higher; for, in imagination, we see it curving and descending to the level of the present surface again. Hence it forms, at the same time, part of one side of a great concave arch, and of a great convex arch, just as every inclined surface on the ground indicates both a hill and a valley. And guided by this principle we can often reconstruct with much probability folds that have been more or less completely swept away by erosion, or that are buried beyond our sight in the earth’s crust.

The arches of the strata are rarely distinctly indicated in the topography, but must be studied where the ground has been partly dissected, as in cliffs, gorges, quarries, etc. They are also, as a rule, far more irregular and complex than they are usually conceived or represented. The wrinkles of our clothing are often better illustrations of rock-folds than the models and diagrams used for that purpose. This becomes self-evident when we reflect that the earth’s crust is exceedingly heterogeneous in composition and structure, and must, therefore, yield unequally to the unequal strains imposed upon it.

The folds or undulations of the strata may be profitably compared with water-waves. In fact, the comparison is so close that they have been not inaptly called rock-waves. Folds, like waves, unless very large, rarely continue for any great distance, but die out and are replaced by others, giving rise to the en echelon or step-like arrangement. The plan of both a wave and a fold is a more or less elongated ellipse, each stratum in a fold being semi-ellipsoidal or boat-shaped. In other words, a normal fold is an elongated mound of concentric strata, being highest at the centre, sloping very gradually toward the ends, and much more abruptly toward the sides.