192 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



The climatic variations during former periods of the earth 

 were discussed by Lyell in considerable detail. He opposed 

 the opinion that climatic changes had been due to the gradual 

 cooling of the earth from an originally molten state, but admitted 

 that during the Tertiary and Diluvial epochs there had been a 

 warmer climate in Europe. During the Secondary epochs 

 reef-corals had inhabited the temperate zones, and in the 

 Carboniferous epoch tree-ferns and other plants indicative of a 

 moist and warm climate had flourished as far north as 75 N. 

 latitude. Lyell traced climatic variations to the varying dis- 

 tribution of land and water, to the influence of ocean currents, 

 to icebergs, and the accumulation of glacier-ice in the polar 

 districts and in the high mountain-chains. He pointed out the 

 geological phenomena characteristic of the Carboniferous epoch 

 the wide distribution of submarine volcanic products and 

 pelagic limestones, the basin-shaped occurrence of the sedi- 

 mentary rocks, the absence of large terrestrial and fresh-water 

 vertebrates, the absence of purely fresh-water deposits, and the 

 insular character of the flora. From all these characteristics, 

 Lyell concluded that the northern hemisphere had been 

 covered during the Carboniferous epoch by an island studded 

 ocean. He then depicted the later epochs, showing that 

 during the Secondary epochs large continents arose in the 

 temperate regions and produced a change of climate; during 

 the Tertiary time the continents in the northern hemisphere 

 became more extensive in the direction of the North Pole, while 

 the Alps, Apennines, and Pyrenees rose as massive mountain- 

 chains, and promoted the gradual approach of the present 

 climatic conditions. 



Lyell, in the earlier editions of the Principles, attributed 

 little importance to the influence of astronomical causes upon 

 terrestrial variations of climate; afterwards he thought these 

 more worthy of consideration. More especially the changes in 

 the eccentricity of the earth's orbit and in the precession of the 

 equinoxes were treated as important climatic factors, and 

 turned to account in the explanation of the Ice Age. 



Having opposed the " Catastrophal Theory " in the first 

 volume of the Principles, Lyell tried to establish the unifor- 

 mity of all natural agencies in past epochs and in the present, 

 and both in the organic and inorganic world. 



The subject-matter of the second volume covers the same 

 ground as Von Hoffs work, but while the German geologist 



