27. The fossil remains of the luxuriant vegetation of this epoch are being discovered today under the ice of the polar regions, as well as in the torrid zone: therefore it is necessary to conclude, that, since vegetation was uniform, the temperature also must have been equally so. The poles were then not covered with ice as now: then the Earth drew its heat from itself, from the central fire which equally heated all the solid bed, then too thin to offer to it successful resistance. This heat was much greater than that conveyed by the solar rays, enfeebled as they were by the density of the atmosphere. Later on, when the central heat could exert only a feeble influence upon the surface, that of the sun preponderate; and the Polar Regions, receiving only oblique rays giving very little heat, became covered with ice. One understands that at the epoch of which we speak, and for a long time after, ice was unknown upon the Earth.
This period has been a very long one, judging from the number and thickness of the coal-beds. *
* In the Bay of Fundy (Nova Scotia), M. Lyell found upon a coal-bed four hundred yards in thickness, sixty-eight different levels, presenting evident traces of many forest soils, the trunks of the trees of which were still garnished with their roots (L. Figuier). Supposing that it takes one thousand years to form each of these levels; it must have taken sixty-eight thousand years to form this coal-bed alone.