Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Astronomic Geology.

The earth as a planet.—Though supremely important to us, the earth is but one of the minor planets attendant upon the sun, and is in no very special way distinguished as a planetary body. Of the eight planets, four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while three, Mars, Venus, and Mercury, are smaller. There are a host of asteroids, but all together they do not equal the mass of the smallest planet. The average mass of the eight planets is more than fifty times that of the earth, while the largest, Jupiter, is more than three hundred times as massive as the earth. The earth’s position in the group is in no sense distinguished. It is neither the outer nor the inner, nor even the middle planet. Even in the minor group to which it belongs, it is neither the outermost nor the innermost member, though in this group it is the largest. Its average distance from the sun is about 92.9 million miles, and this fixes its revolution at 365¼ days, for its period of revolution is directly dependent on its distance from the sun, and is necessarily longer than the revolutions of the inner planets and shorter than those of the outer planets. Its rotation in twenty-four hours is not far different from that of its neighbor Mars, but is much slower than the more distant and larger planets, Jupiter and Saturn, which rotate in about ten hours. Comparison cannot be made with the innermost and outermost planets, because their rotations are not yet satisfactorily determined. The plane of the earth’s revolution lies near the common plane of the whole system, but this is not peculiar, as all of the planets revolve in nearly the same plane. Only a few of the small asteroids depart notably from this common plane. This has an important bearing on theories of the origin of the system, since this close coincidence of the planes of the orbits is not consistent with any haphazard aggregation of the material. Of similar importance is the fact that all of the planets revolve in the same direction and in ellipses that do not depart widely from circles. The eccentricity of the earth’s orbit is only about ¹⁄₆₀. This eccentricity varies somewhat, due to the disturbing influences of the other planets, and this variation has been regarded by some geologists as an influential cause of climatic changes, but its adequacy to produce great effects has been doubted by others. The inclination of the earth’s axis, now about 23½°, holds an intermediate position, some of the planets having axes more inclined, as Saturn, 26⅚°, and others less inclined, as Jupiter, 3°. The inclination of the axis is subject to trivial variations at present, and in the long periods of the past has possibly changed more notably. This possible change has also been thought to be a cause of climatic variation, but its efficiency has not been demonstrated.

Its satellite.—The earth is peculiar in having one unusually large satellite, which has a mass ¹⁄₈₁ of its own. The great planets have several satellites whose combined mass exceeds that of the moon, and perhaps in some few cases the individual satellites may be larger than the moon, but they do not sustain so large a ratio to their planets, for Titan, probably the largest, is only ¹⁄₄₆₀₀ of the mass of Saturn. There is little doubt that the moon has played an important part in the history of the earth. It is the chief agency in developing oceanic tides, and it possibly also develops a body tide in the earth itself. These tides act as a brake on the rotation of the earth and tend to reduce its rate, and thereby to lengthen the day. While this may have been counteracted in some measure by the shrinkage of the earth, which tends to increase its rate of rotation, it has been held by eminent physicists and geologists that the rotation of the earth has been greatly lessened during its history, and that a long train of important consequences has resulted. If the contraction of the earth has been sufficient to offset this lessening, the tidal brake must be credited with the prevention of the excessive speed of rotation which would otherwise have been developed. The tides are efficient agencies in the shore wear of the oceans, and in the distribution of marine sediments, and these, it will be seen later, are important elements in the formation of strata.

Dependence on the sun.—By far the most important external relation of the earth, however, is its dependence on the sun. The earth is a mere satellite of the sun, less than ¹⁄₃₀₀₀₀₀ of its mass, and hence under its full gravitative control. The earth is dependent on the sun for nearly all its heat and light, and, through these, for nearly all of the activities that have given character to its history. It is too much to say that all activities on the surface of the earth are solely dependent on those of the sun, for a certain measure of heat and light and other energy is derived from other bodies, and a certain not inconsiderable source of energy is found in the interior of the earth itself; yet all of these are so far subordinate to that great flood of energy which comes from the sun that they are quite inconsequential. The history of the earth in the past has been intimately dependent upon that of the sun, and its future is locked up with the destiny of that great luminary. Geology in its broadest phases can therefore scarcely be separated from the study of the sun, but this falls within the function of the astronomer rather than the geologist.

Meteorites.—There are a multitude of small bodies passing through space in varying directions and with varying velocities and occasionally encountering the earth, to which they add their substance. Some of these meteorites revolve about the sun much as if they were minute planets, but some of them come from such directions and with such velocities as to show that they do not belong to the solar family. Some consist almost wholly of metal, chiefly iron alloyed with a small percent. of nickel (holosiderites); some consist of metal and rock intimately mixed (syssiderites and sporadosiderites); and some consist wholly of rock (asiderites). The rock is usually composed of the heavier basic minerals, though some meteorites consist largely of carbonaceous material. Besides meteorites, there is little doubt that wandering gaseous particles strike the earth, but this is beyond the reach of present demonstration. The amount of substance added to the earth by these meteorites and gases in recent times is relatively slight compared with the whole body of the earth. What contribution may have come to the earth in earlier times from such sources is a matter of hypothesis which will be discussed later.