Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

That a contracting sphere does indeed pass through such a series of changes has been shown by the behavior of contracting soap bubbles and of rubber balloons, as well as by experiments upon the exhaustion of air contained in hollow metal spheres of only moderate strength. In all these instances, the ultimate form produced indicates an indenting of four sides of the sphere which have the positions of the faces of a tetrahedron. The late Professor Prinz of Brussels secured some extremely interesting results in which he obtained intermediate forms with six angles, but unfortunately these studies were not prepared for publication at the time of his death.

The earth’s departure from the spheroid in the direction of the modified tetrahedron is, as we have seen, no hypothesis, but observed fact revealed in (1) the concentration of the land about a central ocean in the northern hemisphere; in (2) the antipodal relation of the land to the water areas, and in (3) the threefold subdivision of the surface into north and south belts by the two greater oceans and the Caspian Depression.

The earlier figures of the earth.—The manner in which continent and ocean are dovetailed into each other in an east-and-west direction has been generally adduced as additional evidence for the tetrahedral figure as above described. Closer examination shows that instead of being in harmony with this figure, it indicates a departure from it, and, as we shall see, a significant departure which undoubtedly has its origin in the earlier history of the planet. The mediterranean seas of both the eastern and the western hemispheres likewise interfere with the perfection of the tetrahedral figure and require an explanation.

Let us then examine in outline the past history of the world with reference especially to the evolution of the continents and to the times and the manners of surface change. It is now well known that there have been three major periods of great deformation of the earth’s shell. The first of these of which we have record came at the end of the first great era of geologic history, the so-called Eozoic era; a second great transformation came at the close of the second or Paleozoic era; and a third began at the end of the next or Mesozoic era, an adjustment which is apparently continuing to-day. Each of these great surface deformations was accompanied by great volcanic eruptions of which we have the evidence in the lavas remaining for our inspection, and each was followed by the formation of great glaciers which spread over large areas of the existing continents.

Before the earliest of these great changes, the earth appears to have approximated in its figure somewhat closely to the ideal spheroid, for it was everywhere enveloped in the hydrosphere as a universal ocean. Toward the close of this period came the adjustments which brought the lithosphere to protrude through the hydrosphere in shield-like continents whose distribution, as shown by the rocks of this period, is of great significance. Within the northern hemisphere rose three land shields spaced at nearly equal intervals and at nearly equal distances from the northern pole. One of these was centered where now is Hudson Bay, another about the present Baltic Sea, and the relics of the third are found in northeastern Siberia. These earliest continents have been referred to as the Laurentian, Baltic, and Angara shields. Within the southern hemisphere shields appear to have developed in somewhat similar grouping, namely, in South America, in South Africa, and in Australia ([Figs. 3] and [5]).

Fig. 5.—Approximations to earlier and present figures of the earth.

These coigns or angles of a form into which the earlier spheroid of the earth was being transformed have persisted through the greater part of subsequent geologic time, and have been enlarged by the growth of sediments about them as well as by the later elevation and wrinkling of these deposits into marginal mountain ranges.

The continents and oceans which arose at the close of the Paleozoic era.—At the close of the second great era in the recorded history of the earth, the now somewhat enlarged continents were profoundly altered during a series of convulsive movements within the surface shell of the lithosphere. When these convulsions were over, there was a new disposition of land and sea, but one quite different from the present arrangement. Instead of being extended in north-south belts, as they are at present, the continents stretched out in broad east-west zones, one in the northern and the other in the southern hemisphere. To the broad southern continent of which so little now remains, the name “Gondwana Land” has been given, and to the sea which divided the northern from the southern continent the name “Ocean of Tethys.” The northern continent stretched across the site of the present Atlantic Ocean as the “North Atlantis”, its northern shore to the westward being somewhat farther south than the present northern coast of North America, since life forms migrated in the northern ocean from the site of Behring Sea to that of the present North Atlantic.

This arrangement of land and water during the middle period of the earth’s recorded history, when considered with reference both to its earlier and to its later evolution, may perhaps be best accounted for by the assumption that the lithosphere was then shaped like [Fig. 5] (middle). In this figure two truncated tetrahedrons are joined in a common plane of contact which may be described as the twin plane. This medial depression upon the lithosphere was occupied by the intercontinental sea, the Ocean of Tethys.