amount of land. Thus, throughout geological history we should look for cyclic changes in the relative area of the lands within the tropics and similar changes of opposite phase in higher latitudes. The extent of the change would depend upon (a) the amount of alteration in the speed of rotation, and (b) the extent of low land in low latitudes and of shallow sea in high latitudes. According to Slichter's tables, if the earth should rotate in twenty-three hours instead of twenty-four, the great Amazon lowland would be submerged by the inflow of oceanic water, while wide areas in Hudson Bay, the North Sea, and other northern regions, would become land because the ocean water would flow away from them.[78]
Following the prompt equatorward movement of water which would occur as the speed of rotation increased, there must also be a gradual movement or creepage of the solid rocks toward the equator, that is, a bulging of the ocean floor and of the lands in low latitudes, with a consequent emergence of the lands there and a relative rise of sea level in higher latitudes. Tidal retardation would have a similar effect. Suess[79] has described widespread elevated strand lines in the tropics which he interprets as indicating a relatively sudden change in sea level, though he does not suggest a cause of the change. However, in speaking of recent geological times, Suess reports that a movement more recent than the old strands "was an accumulation of water toward the equator, a diminution toward the poles, and (it appears) as though this last movement were only one of the many oscillations which succeed each other with the same tendency, i.e., with a positive excess at the equator, a negative
excess at the poles." (Vol. II, p. 551.) This creepage of the rocks equatorward seemingly might favor the growth of mountains in tropical and subtropical regions, because it is highly improbable that the increase in the bulge would go on in all longitudes with perfect uniformity. Where it went on most rapidly mountains would arise. That such irregularity of movement has actually occurred is suggested not only by the fact that many Cenozoic and older mountain ranges extend east and west, but by the further fact that these include some of our greatest ranges, many of which are in fairly low latitudes. The Himalayas, the Javanese ranges, and the half-submerged Caribbean chains are examples. Such mountains suggest a thrust in a north and south direction which is just what would happen if the solid mass of the earth were creeping first equatorward and then poleward.
A fact which is in accord with the idea of a periodic increase in the oceans in low latitudes because of renewed bulging at the equator is the exposure in moderately high latitudes of the greatest extent of ancient rocks. This seems to mean that in low latitudes the frequent deepening of the oceans has caused the old rocks to be largely covered by sediments, while the old lands in higher latitudes have been left more fully exposed to erosion.
Another suggestion of such periodic equatorward movements of the ocean water is found in the reported contrast between the relative stability with which the northern part of North America has remained slightly above sea level except at times of widespread submergence, while the southern parts have suffered repeated submergence alternating with great emergence.[80] Furthermore, although
the northern part of North America has been generally exposed to erosion since the Proterozoic, it has supplied much less sediment than have the more southern land areas.[81] This apparently means that much of Canada has stood relatively low, while repeated and profound uplift alternating with depression has occurred in subtropical latitudes, apparently in adjustment to changes in the earth's speed of rotation. The uplifts generally followed the times of submergence due to equatorward movement of the water, though the buckling of the crust which accompanies shrinkage doubtless caused some of the submergence. The evidence that northern North America stood relatively low throughout much of geological time depends not only on the fact that little sediment came to the south from the north, but also on the fact that at times of especially widespread epicontinental seas, the submergence was initiated at the north.[82] This is especially true for Ordovician, Silurian, Devonian, and Jurassic times in North America. General submergence of this kind is supposed to be due chiefly to the overflowing of the ocean when its level is slowly raised by the deposition of sediment derived from the erosion of what once were continental highlands but later are peneplains. The fact that such submergence began in high latitudes, however, seems to need a further explanation. The bulging of the rock sphere at the equator and the consequent displacement of some of the water in low latitudes would furnish such an explanation, as would also a decrease in the speed of rotation induced by tidal retardation, if that retardation were great enough and rapid enough to be geologically effective.
The climatic effects of the earth's contraction, which we shall shortly discuss, are greatly complicated by the fact that contraction has taken place irregularly. Such irregularity has occurred in spite of the fact that the processes which cause contraction have probably gone on quite steadily throughout geological history. These processes include the chemical reorganization of the minerals of the crust, a process which is illustrated by the metamorphism of sedimentary rocks into crystalline forms. The escape of gases through volcanic action or otherwise has been another important process.
Although the processes which cause contraction probably go on steadily, their effect, as Chamberlin[83] and others have pointed out, is probably delayed by inertia. Thus the settling of the crust or its movement on a large scale is delayed. Perhaps the delay continues until the stresses become so great that of themselves they overcome the inertia, or possibly some outside agency, whose nature we shall consider later, reënforces the stresses and gives the slight impulse which is enough to release them and allow the earth's crust to settle into a new state of equilibrium. When contraction proceeds actively, the ocean segments, being largest and heaviest, are likely to settle most, resulting in a deepening of the oceans and an emergence of the lands. Following each considerable contraction there would be an increase in the speed of rotation. The repeated contractions with consequent growth of the equatorial bulge would alternate with long quiet periods during which tidal retardation would again decrease the speed of rotation and hence lessen the bulge. The result would be repeated changes of distribution of land and water, with consequent changes in climate.
I. We shall now consider the climatic effect of the repeated changes in the relative amounts of land and water which appear to have resulted from the earth's contraction and from changes in its speed of rotation. During many geologic epochs a larger portion of the earth was covered with water than at present. For example, during at least twelve out of about twenty epochs, North America has suffered extensive inundations,[84] and in general the extensive submergence of Europe, the other area well known geologically, has coincided with that of North America. At other times, the ocean has been less extensive than now, as for example during the recent glacial period, and probably during several of the glacial periods of earlier date. Each of the numerous changes in the relative extent of the lands must have resulted in a modification of climate.[85] This modification would occur chiefly because water becomes warm far more slowly than land, and cools off far more slowly.
An increase in the lands would cause changes in several climatic conditions. (a) The range of temperature between day and night and between summer and winter would increase, for lands become warmer by day and in summer than do oceans, and cooler at night and in winter. The higher summer temperature when the lands are widespread is due chiefly to the fact that the land, if not snow-covered, absorbs more of the sun's radiant energy than does the ocean, for its reflecting power is low. The lower winter temperature when lands are widespread occurs not only because they cool off rapidly but