Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Continent-forming movements.—True continent-forming movements appear to have antedated the earliest known sediments. As far back as we can read the sedimentary record, the continents seem to have been well established, and there is little evidence that they have since been fundamentally changed. It is true that some very eminent geologists have rather freely connected formations on one continent with formations having similar faunas on an opposite continent, by a hypothetical conversion of the intervening ocean-bottoms into land or shallow water; but most such faunal relations can be explained almost equally well by migration around the coasts, or at most by mere ridge-connections. The paucity, if not total absence, of abysmal deposits in the strata of the continents, taken with the persistence of terrestrial and coastal faunas, leaves little room for assigning an interchange of position between abysmal depths and continental elevations, and vice versa. Dynamic considerations also offer grave difficulties. The doctrine of the persistence of continents probably ought not to be pushed so far as to exclude shallow water, or even land, connections between South America, Antarctica, Australia, India, and South Africa, directly or indirectly, at certain stages of geological history. Without forming final conclusions as to the measure of the change which the continents have suffered during known geological history, it is safe to conclude that the continents and ocean-basins were in the main formed very early in the earth’s history, and that subsequent changes have consisted chiefly in the further sinking of the basins and the further protrusion of the land, save as the latter has been cut down by erosion. Incidentally, the ocean-basins have probably been extended and the continents restricted. On the other hand, the continents have been built out on their borders by wash from the land, and the waters of the ocean have been somewhat lifted by the deposition of sediment in their basins. It is estimated that the cutting away of the present continents, and the deposition of the material in the ocean-basins, would raise the sea-level about 650 feet. (R. D. George.)

Relations of these movements in time.—The folding movements seem to have had extraordinary prevalence in the earliest ages, for the Archean rocks are almost universally crumpled, and often in the most intricate fashion. There is no sign that the folding was then limited to the borders of the continents; it seems rather to have affected the whole continental surface. After the beginning of the well-known sedimentary series, crumpling appears to have taken place chiefly at long intervals, thus marking off great time-divisions, and to have been confined at any given stage to certain tracts, chiefly on the borders of great segments of the earth’s crust.

Concerning the plateau-forming movements in the past, knowledge is very meager, as the detection of plateaus of ancient times is more difficult than the detection of folds. Gentle warpings have apparently been in progress at all times.

Relations of vertical to horizontal movements.—The downward movements are unquestionably the primary ones, and the horizontal ones are secondary and incidental. The fundamental feature is doubtless central condensation actuated by gravity, and the master movements are the sinkings of the ocean-basins. The great periodic movements that made mountains and plateaus, and changed the capacity of the ocean-basins, probably started with the sinking of part or all of the ocean-bottoms. In the greater periodic movements, probably all the basins participated more or less, but some seem to have been more active than others. For example, in the last great mountain-making period, the Pacific basin seems to have been more active than the Atlantic, while in the similar great event at the close of the Paleozoic, the opposite seems to have been true. The squeezing up of the continents doubtless took place simultaneously with the settling of the basins. The true conception is perhaps that the ocean-basins and continental platforms are but the surface forms of great segments of the lithosphere, all of which crowd toward the center, the stronger and heavier segments taking precedence and squeezing the weaker and lighter ones between them. The area of the more depressed or master segments is almost exactly twice that of the protruding or squeezed ones. This estimate includes in the latter about 10,000,000 square miles now covered with shallow water. The volume of the hydrosphere is a little too great for the true basins, and it runs over, covering the borders of the continents. The amount of the overflow fluctuates from time to time, and may be neglected in a study of the movements and deformations of the lithosphere.

The squeezed segments.—The great protruding segments show a tendency toward rude triangularity. They are (1) the Eurasian, now strongly ridged on the south and east, and relatively flat on the northwest; (2) the African, rather strongly ridged on the east, but less abruptly elevated on the west and north; (3) the North American, now strongly ridged on the west, more gently on the east, and relatively flat at the north and in the interior; (4) the South American, strongly ridged on the west and somewhat on the northeast and southeast.

The foregoing form the major group. The minor group embraces (5) the Antarctic segment, not as yet sufficiently known to be well defined, and (6) the Australian, broadly reniform rather than triangular. To these are perhaps to be added (7) the largely submerged platform that stretches from Sumatra and Java on the southwest to the Philippines on the northeast, and is attached to India on the northwest; and (8) Greenland, which, though closely associated with North America, is partially separated by a rather deep depression.

The depressed or master segments.—The great sunken segments show a tendency to assume roughly polygonal, rather than triangular, forms. This accords with the primary place assigned them, since, in a spherical surface divided into larger and smaller segments, the major parts should be polygonal while the minor residual segments are more likely to be triangular. The major segments are (1) the Pacific, (2) the Indian, (3) the North Atlantic, and (4) the South Atlantic. These form the principal group, while (5) the Arctic deeps (not including the shallow epicontinental portions), (6) the Mediterranean, (7) the Caribbean, and (8) the chain of deep pits between the Philippine ridge and the Bornean platform, constitute a subordinate group.

Each member of the minor group is an irregular chain of depressed pits rather than a single continuous deep, unless the Arctic depression, of which little is now known, proves an exception. They lie between the greater segments at what may be conceived to be points of critical working relations, and are accompanied by small elevated blocks. The Caribbean, the Mediterranean, and the Bornean regions are the seats of the greatest present volcanic and related activities.

In a general view, there are then four great sunken quadrilaterals and four great elevated triangles, with minor attendants in each class. Lest fondness for simplicity and symmetry lead too far, we must hasten to observe that the dimensions are not alike in either class. The Pacific segment is more than twice the size of any other basin segment, and four times that of the North Atlantic. The Eurasian triangle is more than twice the average size of the other land segments, and nearly three times that of the South American. Nor is there any large common divisor of approximate accuracy. This is not at all strange if the earth be regarded as a body of somewhat heterogeneous composition which naturally shrank in rather irregular segments. On the other hand, this irregularity is somewhat strange if the earth has evolved from a very homogeneous and symmetrical, primitive, fluid state. It is also a serious consideration in any theory that appeals to crystalline form, or analogy, as in the doctrine of a tetrahedral earth.

Roughly approximated in millions of square miles, the major depressed segments are as follows: the Pacific, 60, the Indian, 27, the South Atlantic, 24, and the North Atlantic, 14, leaving 8 for minor depressions. The elevated segments are Eurasian, 24, African, 12, North American, 10, and South American, 9, leaving 10 for the minor blocks.