336 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



have been transported thither hy the glaciers at the time of their great- 

 est extension.* Charpentier developed tbis theory, and to him is due 

 the honor of having introduced into science a fact now generally ad- 

 mitted. 



This new theory was supported by the authority of Professor Agassiz, 

 but immediately there was a division of opinion among glacialists; some 

 believed, with Agassiz, that a Siberian cold reigned throughout all 

 Europe, and that all the northern part was covered with a cap of ice ; 

 others, with Charpentier, admitted only glaciers of gigantic dimensions. 

 Sir Charles Lyell was one of the most active defenders of this theory, 

 which he developed in his numerous works. 



Geologists are not in accord as to the number and duration of the gla- 

 cial periods. Some maintain there was a single period of cold ; others 

 two or several, limited to a certain epoch, beyond which the existence 

 of ice was hardly probable. M. D'Archiact thought that the polar ice did 

 not exist beyond the Tertiary period. However, the fissures or flaws in 

 the Permian, Liassic, and Cretacean deposits, as well as the arrest of 

 animal and vegetable life, prove, it seems to us, that the climate under- 

 went great changes, not only during the Tertiary and Quaternary pe- 

 riods, but also during the above-mentioned formations. Climates have 

 varied very little since the Coal period, for the coal-beds are fouud in 

 the same zone to which turf-marshes are now almost exclusively con- 

 fined. In proportion as we advance toward the south of Europe, says 

 M. Lesquereux, we cease to find coal beyond the limits which contain 

 peai-bogs, and towards the north the thickness of the coal-beds is in 

 relative proportion to the thickness of the peat-deposits in the neighbor- 

 hood. The same fact may be stated for the austral hemisphere, and 

 neither coal nor peat is found between the tropics.} 



It must, however, be admitted that notwithstanding this localization 

 of climates the temperature was less extreme in the two opposed seasons, 

 and there was greater moisture, on account of the preponderance of 

 water in the austral hemisphere. The same thing is repeated in the 

 Tertiary period; the European flora, analogous to that of Kew Orleans, 

 indicates a damp and marshy climate. The temperature, however, could 

 not have been much higher than it is now, or the existence of peat- 

 bogs would have been impossible.§ "We know, moreover, the slowness 

 of the process of the cooling of the crust of the globe. If we calculate 

 the temperature of the latter for a supposed age of the Coal period, or 

 even an earlier, we find that the quantity of heat radiated into celestial 

 space, although necessarily greater than at present, could not sensibly 

 have influenced the modification of climate. As the radiation of the 

 central heat of the earth acted almost under the same condition as to- 



* Favre, Eech. GM., t. i, pp. 57, 178. 



t Cours de PaUont. Strat., t. ii, p. 29. 



t D'Arcbiac, oj). cit., p. 403. 



§ Dr. Oswald Heer, Flora Tertiaria Helvetia;, t. i, p. 12 ; t. iii, p. 324. 



