Some Salient Points in the Science of the Earth

by Sir John William Dawson

(7) The differences in the form and elevation of our continents, and in the consequent distribution of surfaces of different absorbent and radiating power, and of the oceanic currents, are known causes of climatal change, and have been referred to in these papers as competent to account for many, at least, of the phenomena.

(8) Reference has already been made, in connection with the distribution of plants, to the possibility that the primeval atmosphere was richer in carbon than that of more modern times, and that this might operate to produce diminution of radiation, and consequent uniformity of temperature; but this cause could not have been efficient in the later geological periods.

There may thus be said to remain two theories of those enumerated by Wood, to which more detailed consideration may be given, namely, numbers four and seven, which may be named respectively those of Croll and Lyell, or the astronomical and geographical theories.

The late Mr. Croll has, in his valuable work "Climate and Time," and in various memoirs, brought forward an ingenious astronomical theory to account for changes of climate. This theory, as stated by himself, is that when the eccentricity of the earth's orbit is at a high value, and the northern winter solstice is in perihelion, agencies are brought into operation which make the south-east trade winds stronger than the north-east, and compel them to blow over upon the northern hemisphere as far as the Tropic of Cancer. The result is that all the great equatorial currents of the ocean are impelled into the northern hemisphere, which thus, in consequence of the immense accumulation of warm water, has its temperature raised, so that ice and snow must, to a great extent, disappear from the Arctic regions. In the prevalence of the converse conditions the Arctic zone becomes clad in ice, and the southern has its temperature raised.

At the same time, according to Croll's calculations, the accumulation of ice on either pole would tend, by shifting the earth's centre of gravity, to raise the level of the ocean and submerge the land on the colder hemisphere. Thus a submergence of land would coincide with a cold condition, and emergence with increasing warmth. Facts already referred to, however, show that this has not always been the case, but that in many cases submergence was accompanied with the influx of warm equatorial waters and a raised temperature, this apparently depending on the question of local distribution of land and water; and this, in its turn, being regulated not always by mere shifting of the centre of gravity, but by foldings occasioned by contraction, by equatorial subsidences resulting from the retardation of the earth's rotation, and by the excess of material abstracted by ice and frost from the Arctic regions, and drifted southward along the lines of arctic currents. This drifting must in all geological times have greatly exceeded, as it certainly does at present, the denudation caused by atmospheric action at the equator, and must have tended to increase the disposition to equatorial collapse occasioned by retardation of rotation.[178]

[178] Croll, in "Climate and Time," and in a note read before the British Association in 1876, takes an opposite view; but this is clearly contrary to the facts of sedimentation, which show a steady movement of débris toward the south and south-west.

While such considerations as those above referred to tend to reduce the practical importance of Mr. Croll's theory, on the other hand they tend to remove one of the greatest objections against it—namely, that founded on the necessity of supposing that glacial periods recur with astronomical regularity in geological time. They cannot do so if dependent on other causes inherent in the earth itself, and producing important movements of its crust.

Sir Robert Ball has in a recent work very ingeniously improved this theory by showing that Croll was mistaken in assigning equal amounts of heat to the earth, as a whole, in the periods of greater and less eccentricity. This would tend to augment the effect of astronomical revolutions as causes of difference of temperature; but has no bearing on the more serious geological objections to the theory in question.

A fatal objection, however, to Croll's theory, the force of which has been greatly increased by recent discoveries, is that the astronomical causes which he adduces would place the close of the last Glacial period at least 80,000 years ago, whereas it is now certainly known from geological facts that the close of the last Glacial period cannot be older than about an eighth or a tenth of that time. This difficulty seems to have caused the greater number of geologists, specially acquainted with the later geological periods, to regard this theory as quite inapplicable to the facts.