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 August, 1864.

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.

the temperature was free from extremes; the atmosphere being warm and moist, somewhat resembling that of New Zealand and the surrounding islands, which we endeavour to imitate artificially in our hothouses."*

The enormous quantity of the carboniferous vegetation shows also that the climate under which it grew could not have been of a tropical character, or it must have Been decomposed by the heat. Peat, so abundant in temperate regions, is not to be found in the tropics.

The condition most favourable to the preservation of vegetable remains, at least under the form of peat, is a cool, moist, and equable climate, such as prevails in the Falkland Islands at the present day. "In these islands," says Mr. Darwin, "almost every kind of plant, even the coarse grass which covers the whole surface of the land, becomes converted into this substance."†

From the evidence of geology we may reasonably infer that were the difference between our summer and winter temperature nearly annihilated, and were we to enjoy an equable climate equal to, or perhaps a little above, the present mean annual temperature of our island, we should then have a climate similar to what prevailed during the Carboniferous epoch.

But we have already seen that such must have been the character of our climate at the time that the eccentricity of the earth's orbit was at a maximum, and winter occurred when the earth was in the perihelion of its orbit. For, as we have already shown, the earth would in such a case be 14,212,700 miles nearer to the sun in winter than in summer. This enormous difference, along with other causes which have been discussed, would almost extinguish the difference between summer and winter temperature. The almost if not entire absence of ice and snow, resulting from this condition of things, would, as has already been shown, tend to raise the

"Coal Fields of Great Britain," p. 45. Third Edition. + "Journal of Researches," chap. xiii.

mean annual temperature of the climate higher than it is at present.

Conditions necessary for the Preservation of the Coal Plants.But in order to the formation of coal, it is not simply necessary to have a condition of climate suitable for the growth, but also for the preservation, of a luxuriant vegetation. The very existence of coal is as much due to the latter circumstance as to the former; nay more, as we shall yet see, the fact that a greater amount of coal belongs to the Carboniferous period than to any other, was evidently due not so much to a more extensive vegetable growth during that age, suited to form coal, as to the fact that that flora has been better preserved. Now, as will be presently shown, we have not merely in the warm periods of a glacial epoch a condition of climate best suited for the growth of coal plants, but we have also in the cold periods of such an epoch the condition most favourable for the preservation of those plants.

One circumstance necessary for the preservation of plants is that they should have been covered over by a thick deposit of sand, mud, or clay, and for this end it is necessary that the area upon which the plants grew should have become submerged. It is evident that unless the area had become submerged, the plants could not have been covered over with a thick deposit; and, even supposing they had been covered over, they could not have escaped destruction from subaërial denudation unless the whole had been under water. Another condition favourable, if not essential, to the preservation of the plants, is that they should have been submerged in a cold and not in a warm sea. Assuming that the coal plants grew during a warm period of a glacial epoch, we have in the cold period which succeeded all the above conditions necessarily secured.

It is now generally admitted that the coal trees grew near broad estuaries and on immense flat plains but little elevated above sea-level. But that the Lepidodendra, Sigillaria, and other trees, of which our coal is almost wholly composed, grew on dry ground, elevated above sea-level, and not in swamps and