The Elements of Geology; Adapted to the Use of Schools and Colleges

by Justin R. Loomis

But it is doubtful whether there ever is absolute stability of any portion of the surface for long periods of time. Of the minor vertical movements of the interior of continents, there can, from the nature of the case, be no evidence whatever. Changes of level, where they are known to be taking place, are so slow, that they are hardly perceptible in the period of a human life. Such changes had been going on for centuries in Sweden before they were suspected. As accurate observations have increased in number, and historical records become available, it is becoming known that a very large amount of the seaboard is undergoing change of level. It becomes probable, then, that these extremely slow changes of level are constantly and everywhere taking place.

That portion of the crust of the earth constituting the present continents, being further removed from the centre, would part with its heat more rapidly, and receive heat from the central mass more slowly, than that portion which at present constitutes the bed of the sea. The continents are therefore in a situation to undergo contraction and depression, and the bed of the sea is most favorably situated for rising. If the distribution of water through the mass has any influence in promoting its expansion, then the bed of the sea would receive this supply most abundantly, and the continents the least so. We see, then, in nature, those provisions for an alteration of level, which, from the character of the several rock formations, we know to have taken place. When any portion of the earth’s surface is covered with the sea, the conditions exist which will at length elevate it. When it becomes dry land, the conditions exist which will in time depress it below the level of the ocean. Hence, those impressions in regard to the land, as stable beyond the possibility of change, we ought to abandon; and those vertical movements, which, when proved, we are accustomed to regard as extraordinary, we shall, at length, consider as only particular instances of one of the most general laws of nature.

7. Variations of Climate.—The only sources of heat by which climate can be affected are the sun and the heated interior of the earth.

If the former melted condition of the entire mass of the earth be assumed, the temperature of the surface must have been increased, by conduction of heat from within, for long periods after the superficial stratum had become solid. It is, however, susceptible of proof, that the present climates are not sensibly affected by interior heat, though at a little more than a mile below the surface the temperature is equal to that of boiling water. At any time, therefore, after the waters had become condensed, collected into oceans, and become sufficiently cool to support the animal life of which the remains are now found, it is not probable that the climate was, to any considerable extent, influenced by the heat conducted from the interior.

Still, there have been great changes of climate since those early organic forms existed; and, since we have no ground for supposing that the temperature of the sun’s rays has suffered any reduction, we have to inquire whether the means of retaining the heat from the sun could at any time have been different. The relative position of land and water depends, as we have seen, upon igneous causes, and has been very different at different times. We shall find that climate must have been greatly modified by these changes; for the land radiates and absorbs heat freely, and water possesses this power in a very low degree.

Let us suppose the zone comprised between the tropics to be occupied by land, and the portions without these limits to be covered with water. Under these conditions, the land, having a nearly vertical sun the whole time, would accumulate heat to a degree scarcely compatible with the existence of animal life. This is sufficiently proved by the oppressive tropical climates of the present time, influenced as they are by polar lands and contiguous seas.

Under the same conditions, the sea would be heated by contact with the land, and the heat would be distributed by marine currents to the polar regions. But the water thus distributed would not part with its heat, because it has but little radiating power, and nowhere comes in contact with polar land. It follows, then, that both land and water would be subjected to a very high temperature.

But, if we suppose the land confined to the polar regions, and the sea to the equatorial, the opposite results would follow. The equatorial sea would absorb but a small proportion of the solar heat which would be thrown upon it. The land would receive the sun’s rays too obliquely to receive much elevation of temperature, as the present polar climates show. Hence, the temperature of the earth would differ but little from that of the planetary spaces, which is fifty-eight degrees below zero, a temperature too low to allow of any considerable development of organic life.

These are the conclusions to which we are led by considering the different powers of land and water to absorb and radiate heat, and we shall find that the existing climates are in accordance with these conclusions. America has a lower temperature than Europe in the same latitudes. It has also a smaller proportion of land in the equatorial regions, and a greater proportion in the north polar regions. The eastern continent is colder in Asia than in Europe in the same latitudes. It has also less equatorial and more polar land. The southern is colder than the northern hemisphere at equal distances from the equator. There is also less land near the equator on the south side, and probably as much land around the south as the north pole.

Hence, we see that there may have been such a relation of land and water as to account for all the variations of temperature which are known to have existed. We cannot say that such actually has been the case. We can tell, with some degree of accuracy, what portions of the present continents were land at the several geological periods; but three-fourths of the surface of the earth is covered with water, and of the condition of this portion during those periods we have no means even of conjecturing. We can only say, that, by the operation of known causes, the relative position of land and water may have been such as to produce the climates known to have existed at former periods of the history of the earth.