At the same time there is a pretty general consensus of the best and latest geologists, that, as Lyell himself suggested, elevation and depression and other geographical changes, though the principal, are not the sole causes of the glacial period. The main argument is, that the phases of this period, though not exactly simultaneous over the whole world, are too nearly so to be due to mere local movements, and require the intervention of some general cosmic cause. We have already seen that of such causes there is none which appears feasible except Croll's theory of the effects of precession combined with high eccentricity.
Let us consider what this theory really asserts. If the earth were a perfect sphere, its orbit round the sun a perfect circle, and the equator coincided with the ecliptic, there would be no seasons. The four quarters of the year would each receive the same quantity of solar heat and light, and the days and nights would be always equal. But the inclination of the equator to the ecliptic, that is, of the earth's plane of daily rotation to that of its annual revolution, necessitates seasons. Each pole must be alternately turned to and away from the sun every year. Each hemisphere, therefore, must have alternately its spring, summer, autumn, and winter. But if the earth's orbit were exactly circular, these seasons would be of equal duration, and the distance from the sun no greater in one than in another. But the earth's orbit is not circular, but elliptic, and the eccentricity, or deviation of the oval from the circular form, varies considerably over very long periods, though always coming back to the amount from which it started. These variations are due to perturbations from the other bodies of the solar system acting according to the law of Newton's gravitation, and therefore calculable.
Again, the earth is not a perfect sphere, but a spheroid, and there is a factor called precession, due to the attraction on the protuberant mass at and towards the equator. The effect of this is, that instead of the earth's axis pointing uniformly towards the same celestial pole, it describes a small circle round it. This circle is completed in about 21,000 years, so that if the earth is nearest to the sun when the North Pole is turned away from it, and it is winter in the northern hemisphere, as is now the case, in 10,500 years the conditions will be reversed, and the southern hemisphere will be in perihelion, or nearest the sun, when its pole points away from it. And as the perihelion portion of the earth's orbit is, owing to its eccentricity, shorter and more quickly traversed than the aphelion portion, this means practically that winters will be shorter than summers in the hemisphere which precession favours, and longer in that to which it is adverse.
As precession now favours the northern hemisphere, which is warmer than the southern in corresponding latitudes, it might be thought at first sight that this was the cause of the glacial period. But it is evident that this is not the case, for the precessional revolutions come round far too rapidly, and it is impossible to suppose that there have been glacial and genial periods alternating every 10,500 years, with all the inevitable changes of seas and lands, and of fauna and flora, accompanying each alternation throughout the whole of geological time. In fact, it is abundantly evident, on historical evidence alone, that there has been no approach to any such changes during the last 10,500 years, which carries us back to a period when our northern summers were short and our winters long.
But Croll's theory brings in the secular variation of the eccentricity, and contends that although precession may have little or no effect while the earth's orbit is nearly circular, as it is now, it must have a considerable effect when the orbit flattens out, so that the distances from the sun and the durations of summer and winter become exaggerated. Croll calculated the periods when such maxima and minima of eccentricity occurred for several revolutions back from the formula of the great astronomer Leverrier, and found that going back for the last 260,000 years there had been two maxima of high eccentricity, one 100,000 years, and the other, and more intense, 210,000 years ago, with corresponding minima of low eccentricity between, which corresponded remarkably well with the refrigeration commencing in the Pliocene, culminating towards its close or in the early Quaternary, subsiding into a long inter-glacial period, rising again in the later Quaternary to a second glacial maximum a little less intense than the first, and finally gradually subsiding into the low eccentricity and temperate climates of more recent times; especially as the geological evidence shows many minor oscillations of heat and cold and advances and retreats of glaciers during each phase of these periods, such as must have occurred from the shorter recurrent effects of precession according to Croll's theory.
Croll's calculations show that, at the period of maximum eccentricity 210,000 years ago, the earth would have been in mid-winter 8,736,420 miles further from the sun than it is now, and the winter half of the year nearly twenty-eight days longer than the summer half, instead of being six days shorter as at present. It appears, moreover, from a volume just published, On the Astronomical Causes of an Ice Age, by Sir R. Ball, one of the highest authorities on mathematics and astronomy, that Croll had understated his case. Ball says that "Croll, misled by a statement of Herschell's, had assumed the number of units of heat received from the sun, in a hemisphere of the earth, as equal in summer and winter. But in reality, of 100 such units, 63 are received in summer and only 37 in winter. As the maximum of eccentricity which is possible would produce an inequality between summer and winter of 33 days, they had the following possible conditions in a hemisphere—summer 199 days and winter 166 days, or summer 166 days and winter 199 days. In each case it must be borne in mind that 63 heat units arrived in summer and 37 in winter. If the summer were a long one and the winter short, then the allotment of heat between the two seasons would be fairly adjusted. The 63 units were distributed over 199 days and the 37 units over 166 days, and a general inter-glacial state was the result on the hemisphere. If, however, a torrent of heat represented by 63 units was poured in during a brief summer of 166 days, whilst, the balance of 37 units is made to stretch itself over 199 days, a brief, intensely hot summer would be followed by a very long and cold winter, and as this condition lasted for many centuries, it seemed sufficient to produce a glacial epoch."
It would be going, too far, however, to assume that these conditions necessarily produced glacial periods whenever they occurred, and Ball himself points out that even on astronomical grounds, several conditions must concur before high eccentricity alone would affect climate. But even with this reservation the same objection applies to assigning, this as the sole or principal cause of Ice Ages, as to precession alone, viz. that periods of high eccentricity occur too frequently to allow us to suppose that every such period in the past has had its corresponding glacial period. There was a maximum phase of eccentricity 700,000 years ago, even higher than that of 210,000 years, and there must have been at least two or three such maxima within each of the twenty-eight geological ages. But there are only two or three traces of glacial periods in past epochs on which geologists can rely with confidence, as proving extensive ice-action—one in the Permian, the other in the Carboniferous age.
There are a few other instances which look like glacial action, as the conglomerate of the Superga at Turin, the Flysch of Switzerland, the great conglomerate at the base of the Devonian; and Professor Geikie thinks that the oldest Cambrian rocks in the West Highlands have been rounded and smoothed by ice before the Silurian strata were deposited on them. But even if these were authenticated and proved to be due to general and not merely local causes, they would not supply anything like the number of glacial periods required by Croll's theory. Croll attempts to meet this by the extensive denudation which has repeatedly carried away such large portions of land surface; but this scarcely explains the absence of the boulders of hard rocks, which accompany every moraine and iceberg; and still less the continuance of the same fauna and flora throughout whole geological periods with little or no change. We have no such abrupt changes as during the last glacial period, when at one time the canary laurel flourished in Central France, while at another the reindeer moss and Arctic willow extended to the Pyrenees, both occurring within what may be called a short time, geologically speaking. On the contrary, there seems to have been no material changes in the flora throughout very long geological periods such as that of the Coal Measures.
The only real answer to this objection is that the question is, not whether Croll's theory is the sole or even the principal cause of glacial periods, or able to influence them materially if the geographical conditions favour genial climates; but whether it has not a co-operating effect, when these conditions are such as to produce glaciation. It seems difficult to suppose that such contrasts of conditions as are pointed out by Sir R. Ball can have had no perceptible effect on climates; or that such close coincidences as are shown between the astronomical theory and geological facts, during the last glacial period, can be due to mere accident.
Geology shows six phases of this period:—(1) a refrigeration coming on in the Pliocene; (2) its culmination in a first and most intense maximum; (3) a gradual return to a milder inter-glacial period; (4) a second refrigeration; (5) its culmination in a second maximum; (6) a second return to genial conditions, such as still prevail. Croll's theory shows six astronomical phases, corresponding to these six geological phases. Geology shows that each of its six phases involves several minor alternations of hot and cold; Croll shows that this must have been the case owing to the effects of the shorter cycles of precession, occurring during the long cycles of variations in eccentricity. Geology tells us that cold alone would not account for a glacial period; we must have heat to supply the evaporation which is condensed by the cold; Croll shows that with high eccentricity cold and long winters must have been accompanied by short and hot summers. And Sir R. Ball's recent calculations show that the argument is really very much stronger than Croll puts it.