Some Salient Points in the Science of the Earth

by Sir John William Dawson

We are thus obliged to fall back upon the old Lyellian theory of geographical changes, with such modifications as recent discoveries have rendered necessary. Taking this as our guide, we reach at once the important conclusion that the movements and distribution of animals and plants, however dependent on climate, altitude and depth, have, when regarded in connection with geological time, been primarily determined by those great movements of the crust of the earth which have established our islands, continents and ocean depths. These geographical changes have also in connection with animal and vegetable growth, deposition of sediments and volcanic ejections, fixed even the stations, soils and exposures of plants and animals. Thus, subject to those great astronomical laws which regulate the temperature of our planet as a whole, our attention may be restricted to the factors of physical geography itself. We must, however, carry with us the idea that though the great continents and the ocean depths may have been fixed throughout geological time, their relative elevations, and consequently their limits, have varied to a great extent, and are constantly changing.

We must also remember that something more than mere cold is necessary to produce a glacial period. It has sometimes been assumed that the tendency of an exceptionally cold winter would necessarily be to accumulate so great a quantity of snow and ice, that these could not be removed in the short though warm summer, and so would go on accumulating from year to year. Actual experience and observation do not confirm this supposition. In those parts of North America which have a long and severe winter, the amount of snow deposited is not in proportion to the lowness of the temperature, but, on the contrary, the greatest precipitation of snow takes place near the southern margin of a cold area, and the snow disappears with great rapidity when the spring warmth sets in. Nor is there, as has been imagined, any tendency to the production of fogs and mists which have been invoked as agencies to shield the snow from the sun. In North America the melting snow is ordinarily carried off as liquid water, or as invisible vapour, and the sky is usually clear when the snow is melting in spring. It is only when warm and moist winds are exceptionally thrown upon the snow-covered land that clouds are produced; and when this is the case, the warm rain that ensues promotes the melting of the snow. Thus there is no possibility of continued accumulations of snow on the lower parts of our continents, under any imaginable conditions of climate. It is only on elevated lands in high latitudes and near the ocean, like Greenland and the Antarctic continent, that such permanent snow-clad conditions can occur, except on mountain tops. Wallace and Wœickoff[179] very properly maintain, in connection with these facts, that permanent ice and snow cannot under any ordinary circumstances exist in low lands, and that high land and great precipitation are necessary conditions of glaciers. The former, however, attaches rather too much importance to snow and ice as cooling agents; for though it is true that they absorb a large amount of heat in passing from the solid to the liquid state, yet the quantity of snow or ice to be melted in spring is so small in comparison with the vast and continuous pouring of solar heat on the surface, that a very short time suffices for the liquefaction of a deep covering of snow. The testimony of Siberian travellers proves this, and the same fact is a matter of ordinary observation in North America.

[179] Von Wœickoff has very strongly put these principles in a Review of Croll's recent book, "Climate and Cosmology"; American Journal of Science, March, 1886.

Setting aside, then, these assumptions, which proceed from incorrect or insufficient information, we may now refer to a consideration of the utmost importance, and which Mr. Croll himself, though he adduces it only in aid of the astronomical theory of glacial periods, has treated in so masterly a manner, as really to give it the first place as an efficient cause. This is the varying distribution of ocean currents, in connection with the differences in the elevation and distribution of land. The great equatorial current, produced by the action of the solar heat on the atmosphere and the water, along with the earth's rotation, is thrown, by opposing continental shores, northward into the Atlantic and Pacific in the Gulf Stream and Japan current, giving us a hot-water apparatus which effectually raises the temperature of the whole northern hemisphere, and especially of the western sides of the continents. Mr. Croll imagined that if his astronomical causes could, to ever so small an extent, intensify the action of these currents, or their determination to the north, we should have a period of warmth, while a similar advantage given to the southern hemisphere would produce a glacial age in the north. But this requires us to assume what ought to be proved; namely, that the position of aphelion, and the increase or decrease of eccentricity, would actually so swing the equatorial current to the north or south. It further requires us to assume—and this is the most important defect of the theory—that no change occurs in the distribution of land and water; because any important change of this kind might obviously exert a dominant influence on the currents. Let us take two examples in illustration of this.

At the present time the warm water thrown into the North Atlantic, co-operating with the prevalent westerly winds, not only increases the temperature of its whole waters, but gives an exceptionally mild climate to western Europe. Still the countervailing influence of the Arctic currents and the Greenland ice, is sufficient to permit numerous icebergs to remain unmelted on the coast of Labrador and Newfoundland throughout the summer. Some of the bergs which creep down to the mouth of the Strait of Belle-Isle, in the latitude of the south of England, actually remain unmelted till the snows of a succeeding winter fall upon them. Now let us suppose that a subsidence of land in tropical America were to allow the equatorial current to pass through into the Pacific. The effect would at once be to reduce the temperature of Norway and Britain to that of Greenland and Labrador at present, while the latter countries would themselves become colder. The northern ice, drifting down into the Atlantic, would not, as now, be melted rapidly by the warm water which it meets in the Gulf Stream. Much larger quantities of it would remain undissolved in summer, and thus an accumulation of permanent ice would take place, along the American coast at first, but probably at length even on the European side. This would still further chill the atmosphere, glaciers would be established on all the mountains of temperate Europe and America, the summer would be kept cold by melting ice and snow, and at length all eastern America and Europe might become uninhabitable, except by Arctic animals and plants, as far south as perhaps 40° of north latitude. This would be simply a return of the glacial age. I have assumed only one geographical change; but other and more complex changes of subsidence and elevation might take place, with effects on climate still more decisive.[180]

[180] According to Bonney, the west coast of Wales is about 12° above the average for its latitude, and if reduced to 12° below the average, its mountains would have large glaciers. So near is England even now to a glacial age.

We may suppose an opposite case. The high plateau of Greenland might subside, or be reduced in height, and the opening of Baffin's Bay might be closed. At the same time the interior plain of America might be depressed, so that, as we know to have been the case in the Cretaceous period, the warm waters of the Mexican gulf might circulate as far north as the basins of the present great American lakes. In these circumstances there would be an immense diminution of the sources of floating ice, and a correspondingly vast increase in the surface of warm water. The effects would be to enable a temperate flora to subsist in Greenland, and to bring all the present temperate regions of Europe and America into a condition of subtropical verdure.

It is only necessary to add that we actually know that changes not dissimilar from those above sketched have really occurred in comparatively recent geological times, to enable us to perceive that we can dispense with all other causes of change of climate, though admitting that some of them may have occupied a secondary place. This will give us, in dealing with the distribution of life, the great advantage of not being tied up to definite astronomical cycles of glaciation, which do not well agree with the geological facts, and of correlating elevation and subsidence of the land with changes of climate affecting living beings. It will, however, be necessary, as Wallace well insists, that we shall hold to a certain fixity of the continents in their position, notwithstanding the submergences and emergences which they have experienced.

Sir Charles Lyell, more than forty years ago, published in his "Principles of Geology" two imaginary maps, which illustrate the extreme effects of various distribution of land and water. In one, all the continental masses are grouped around the equator. In the other they are all placed around the poles, leaving an open equatorial ocean. In the one case the whole of the land and its inhabitants would enjoy a perpetual summer, and scarcely any ice could exist in the sea. In the other, the whole of the land would be subjected to an Arctic climate, and it would give off immense quantities of ice to cool the ocean. Sir Charles remarks on the present apparently capricious distribution of land and water, the greater part being in the northern hemisphere, and, in this, placed in a very unequal manner. But Lyell did not suppose that any such distribution as that represented in his maps had actually occurred, though this supposition has been sometimes attributed to him. He merely put what he regarded as an extreme case to illustrate what might occur under conditions less exaggerated. Sir Charles, like all other thoughtful geologists, was well aware of the general fixity of the areas of the continents, though with great modifications in the matter of submergences and of land conditions. The union, indeed, of these two great principles of fixity and diversity of the continents lies at the foundation of theoretical geology.

We can now more precisely indicate this than was possible when Lyell produced his "Principles," and can reproduce the conditions of our continents in even the more ancient periods of their history. An example of this may be given from the American continent, which is more simple in its arrangements than the double continent of Eurasia. Take, for instance, the early Devonian or Erian period, in which the magnificent flora of that age, the earliest certainly known to us, made its appearance. Imagine the whole interior plain of North America submerged, so that the continent is reduced to two strips on the east and west, connected by a belt of Laurentian land on the north. In the great mediterranean sea thus produced, the tepid water of the equatorial current was circulated, and it swarmed with corals, of which we know no less than 150 species, and with other forms of life appropriate to warm seas. On the islands and coasts of this sea was introduced the Erian flora, appearing first in the north, and with that vitality and colonizing power of which, as Hooker has well shown, the Scandinavian flora is the best modern type, spreading itself to the south. A very similar distribution of land and water in the Cretaceous age gave a warm and equable climate in those portions of North America not submerged, and coincided with the appearance of the multitude of broad-leaved trees of modern types which appeared in the middle Cretaceous, and prepared the way for the mammalian life of the Eocene.