W J McGee

“ Hence, if the purely physical agencies alone are capable of causing glaciation, Dr. Croll's theory of secular changes in climate will stand, whether or not the wind theory of oceanic circulation is correct; but, if they are not capable of producing a glacial period alone, the theory will fall, even if the wind theory of oceanic circulation be correct. This can not be too strongly emphasized. Yet, not only in "Climate and Time," but in his other publications on the subject, Dr. Croll dwells upon the deflection-of ocean-currents as the principal telluric element in his theory! ”

“ Croll then proceeds to show how the accumulation of ice in polar regions would affect the general oceanic circulation: "Owing to the difference between the temperature of the equator and the poles there is a constant flow of air from the poles to the equator. It is to this that the trade-winds owe their existence. Now, as the strength of these winds, as a general rule, will depend on the difference of temperature that may exist between the equator and higher latitudes, it follows that the trades on the cold hemisphere will be stronger than those on the warm. ”

“ If the general series of marine deposits in the Arctic regions were as nearly unfossiliferous as are the sedimentary strata of tropical India, such an hypothesis would be a little more likely to find acceptance among geologists. The fact developed by Meech, that the polar regions ought to have a warmer summer than the equator, if the solar intensity is a fair criterion, would indicate that these regions should have only a temperate climate if the ice were removed and the summer's heat stored up in the earth ”

“ Those who attribute the difference in density to difference in temperature. Lieutenant Maury's theory, stated in his "Physical Geography of the Sea," is rejected because that eminent meteorologist recognized both of the above-named causes of difference in density, while they are very nearly equal and quite antagonistic, and because the actual differences in specific gravity due to these causes are mathematically demonstrable to be incompetent to produce so powerful currents; being, in fact, as Dr. Croll pointedly insists, only one seventh of that necessary to produce the slightest motion. ”

“ Moreover, Professor Dana has shown in his "Journal" that the distribution of ice during the glacial period coincided in a general way with the present distribution of rainfall in the same latitudes. Now, the greater part of the moisture of the United States, especially of the great Mississippi Basin, is derived from the Gulf of Mexico; and the stoppage of the South Atlantic feeder of the Gulf Stream would cool the waters of the Gulf so considerably as to materially diminish our vapor-supply, and at the same time the distribution would be altered. ”

“ Moreover, two important elements in the problem of oceanic circulation are disregarded by Dr. Croll: 1. The water borne from equatorial to polar regions by aerial currents, in the form of aqueous vapor, must exercise a powerful influence not only on the earth's temperature but also upon marine currents; for, as shown by Lieutenant Maury, it is sufficient to permanently render the Arctic seas much less saline than those of tropical regions. ”

“ Sir John Herschel and others, however, soon after demonstrated that the light and heat received by any portion of the earth's surface during any year is practically invariable, whatever the eccentricity of the terrestrial orbit; the greater proportionate length of winter in the hemisphere whose winters occur in aphelion being exactly counterbalanced by the greater proximity of the sun in summer. ”

“ The present eccentricity (0·0168) is such that there is a difference of about eight days in the length of summer and winter in either hemisphere, when the solstices coincide with the apsides; and, the winter solstice of the southern hemisphere being now not far from aphelion, that hemisphere has the long winter and short summer. It is admitted, however (even too readily, it would seem) , that the present degree of eccentricity is too insignificant to exercise much influence on the climate of the globe ”

“ So effective are ocean-currents in distributing the heat of tropical and the cold of polar regions, that our author concludes that the globe would not be habitable for existing orders of beings were their influence to cease.
The heat-conveying power of aërial currents is also discussed; but it seems possible that Dr. Croll has under-estimated their capacity, many times increased as it is by the aqueous vapor with which they are laden. Space will not, however, permit the discussion of this interesting point. ”

“ It seems very doubtful, too, even if the melting of the ice took place with the greatest conceivable rapidity, whether terrestrial animals or plants would spread over the barren wastes of crude glacial débris so rapidly as to people so wide a zone in the brief period assigned. Some of the intercalated fossiliferous beds of the drift, too, are very rich in numbers as well as species of both animals and plants—the latter sometimes forming extensive deposits of lignite—which must have required an immense time for their development. ”

“ Croll points out, be likely to be the first out of all of the ocean-currents to be deflected, it may reasonably be doubted whether any were seriously affected by the cause specified. It must be admitted, however, that the physical agencies which have already been described seem competent to inaugurate a glacial epoch during any period of high eccentricity of the terrestrial orbit, without any assistance from the meteorological influences. ”

“ In seeking for demonstrative evidence of the correctness of the wind theory. Dr. Croll seems to fall into an error which he has repeatedly had occasion to point out in others. Thus, in discussing one of Dr. Carpenter's sections, he shows, from Professor Muncke's coefficients of the density of sea-water, at various temperatures, that the surface of the Atlantic is, at north latitude 23° 10′, two feet six inches, and at north latitude 37° 54′, fully three feet six inches higher than at the equator, on account of the greater thickness at these points of the upper layer of warm water. ”

“ Besides, such a collocation affords a sufficient period for the development of the rich and characteristic fauna and flora of the forest-bed—a widespread fossiliferous and carbonaceous deposit that must represent a lapse of time of geological importance. ”