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But even supposing it could be shown that a change in the obliquity of the ecliptic to the extent assumed by Mr. Belt and Lieutenant-Colonel Drayson would produce a glacial epoch, still the assumption of such a change is one which physical astronomy will not permit. Mr. Belt does not appear to dispute the accuracy of the methods by which it is proved that the variations of obliquity are confined within narrow limits; but he maintains that physical astronomers in making their calculations have left out of account some circumstances which materially affect the problem. These, according to Mr. Belt, are the following:-(1) Upheavals and subsidences of the land which may have taken place in past ages. (2) The unequal distribution of sea and land on the globe. (3) The fact that the equatorial protuberance is not a regular one, "but approaches in a general outline to an ellipse, of which the greater diameter is two miles longer than the other." (4) The heaping up of ice around the poles during the glacial period.

We may briefly consider whether any or all of these can sensibly affect the question at issue. In reference to the lastmentioned element, it is no doubt true that if an immense quantity of water were removed from the ocean and placed around the poles in the form of ice it would affect the obliquity of the ecliptic; but this is an element of change which is not available to Mr. Belt, because according to his theory the piling up of the ice is an effect which results from the change of obliquity.

*

In reference to the difference of two miles in the equatorial diameters of the earth, the fact must be borne in mind that the longer diameter passes through nearly the centre of the great depression of the Pacific Ocean, whereas the shorter diameter passes through the opposite continents of Asia and America. Now, when we take into consideration the fact that these continents are not only two-and-a-half times denser than the ocean, but have a mean elevation of about 1,000 feet above the sealevel, it becomes perfectly obvious that the earth's mass must

* The longer diameter passes from long. 14° 23′ E. to long. 165° 37′ W.

be pretty evenly distributed around its axis of rotation, and that therefore the difference in the equatorial diameters can exercise no appreciable effect on the change of obliquity. It follows also that the present arrangement of sea and land is the best that could be chosen to prevent disturbance of motion.

That there ever were upheavals and depressions of the land of so enormous a magnitude as to lead to a change of obliquity to the extent assumed by Lieutenant-Colonel Drayson and Mr. Belt is what, I presume, few geologists would be willing to admit. Suppose the great table-land of Thibet, with the Himalaya Mountains, were to sink under the sea, it would hardly produce any sensible effect on the obliquity of the ecliptic. Nay more; supposing that all the land in the globe were sunk under the sea-level, or the ocean beds converted into dry land, still this would not materially affect obliquity. The reason is very obvious. The equatorial bulge is so immense that those upheavals and depressions would not to any great extent alter the oblate form of the earth. The only cause which could produce any sensible effect on obliquity, as has already been noticed, would be the removal of the water of the ocean and the piling of it up in the form of ice around the poles; but this is a cause which is not available to Mr. Belt.

Sir Charles Lyell's Theory.-I am also unable to agree with Sir Charles Lyell's conclusions in reference to the influence of the obliquity of the ecliptic on climate. Sir Charles says, "It may be remarked that if the obliquity of the ecliptic could ever be diminished to the extent of four degrees below its present inclination, such a deviation would be of geological interest, in so far as it would cause the sun's light to be disseminated over a broader zone inside of the arctic and antarctic circles. Indeed, if the date of its occurrence in past time could be ascertained, this greater spread of the solar rays, implying a shortening of the polar night, might help in some slight degree to account for a vegetation such as now characterizes lower latitudes, having had in the Miocene and Carboniferous periods a much wider range towards the pole."*

* "Principles," vol. i., p. 294. Eleventh Edition.

The effects, as we have seen, would be directly the reverse of what is here stated, viz., the more the obliquity was diminished the less would the run's rays spread over the arctic and antarctic regions, and conversely the more the obliquity was increased the greater would be the amount of heat spread over polar latitudes. The farther the sun recedes from the equator, the greater becomes the amount of heat diffused over the polar regions; and if the obliquity could possibly attain its absolute limit (90°), it is obvious that the poles would then be receiving more heat than the equator is now.

CHAPTER XXVI.

COAL AN INTER-GLACIAL FORMATION.

Climate of Coal Period Inter-glacial in Character.-Coal Plants indicate an Equable, not a Tropical Climate.-Conditions necessary for Preservation of Coal Plants.-Oscillations of Sea-level necessarily implied.-Why our Coalfields contain more than One Coal-seam.-Time required to form a Bed of Coal.-Why Coal Strata contain so little evidence of Ice-action.-Land Flat during Coal Period.--Leading Idea of the Theory.-Carboniferous Limestones.

An Inter-glacial Climate the one best suited for the Growth of the Coal Plants.-No assertion, perhaps, could appear more improbable, or is more opposed to all hitherto received theories, than the one that the plants which form our coal grew during a glacial epoch. But, nevertheless, if the theory of secular changes of climate, discussed in the foregoing chapters, be correct, we have in warm inter-glacial periods (as was pointed out several years ago)* the very condition of climate best suited for the growth of those kinds of trees and vegetation of which our coal is composed. It is the generally received opinion among both geologists and botanists that the flora of the Coal period does not indicate the existence of a tropical, but a moist, equable, and temperate climate. "It seems to have become," says Sir Charles Lyell, "a more and more received opinion that the coal plants do not on the whole indicate a climate resembling that now enjoyed in the equatorial zone. Tree-ferns range as far south as the southern parts of New Zealand, and Araucanian pines occur in Norfolk Island. A great preponderance of ferns and lycopodiums

*Phil. Mag. for Angust, 1854.

indicates moisture, equability of temperature, and freedom from frost, rather than intense heat."*

Mr. Robert Brown, the eminent botanist, considers that the rapid and great growth of many of the coal plants showed that they grew in swamps and shallow water of equable and genial temperature.

"Generally speaking," says Professor Page, "we find them resembling equisetums, marsh-grasses, reeds, club-mosses, treeferns, and coniferous trees; and these in existing nature attain their maximum development in warm, temperate, and subtropical, rather than in equatorial regions. The Wellingtonias of California and the pines of Norfolk Island are more gigantic than the largest coniferous tree yet discovered in the coal-measures."+

The Coal period was not only characterized by a great preponderance over the present in the quantity of ferns growing, but also in the number of different species. Our island possesses only about 50 species, while no fewer than 140 species have been enumerated as having inhabited those few isolated places in England over which the coal has been worked. And Humboldt has shown that it is not in the hot, but in the mountainous, humid, and shady parts of the equatorial regions that the family of ferns produces the greatest number of species.

"Dr. Hooker thinks that a climate warmer than ours now is, would probably be indicated by the presence of an increased number of flowering plants, which would doubtless have been fossilized with the ferns; whilst a lower temperature, equal to the mean of the seasons now prevailing, would assimilate our climate to that of such cooler countries as are characterized by a disproportionate amount of ferns." +

"The general opinion of the highest authorities," says Professor Hull, " appears to be that the climate did not resemble that of the equatorial regions, but was one in which

"Elementary Geology," p. 399.

+ "The Past and Present Life of the Globe," p. 102.

"Memoirs of the Geological Survey," vol. ii., Part 2, p. 404.

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