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THE WORLD OF LIFE 



maximum effect. The heat thus produced would be greatest 

 where the under surface of the crust was most irregular, that 

 is, under the land surfaces, and especially under the ^' roots of 

 mountains " projecting below the general level. Their cumu- 

 lative results would, therefore, add to the upward forces pro- 

 duced by contraction along lines of weakness.^ 



But whether the various forces here suggested have been 

 the only forces in operation or not, the fact of the repeated 

 slow elevations and depressions of the earth's surface is un- 

 doubted. The most general phenomenon seems to have been 

 the very slow elevation of gTcat beds of strata, deposited one 

 above another along the coasts of a continental mass, or some- 

 times along the shores of inland seas; immediately followed 

 by a process of denudation of this surface by rain and rivers, 

 which, as the elevation continued, carved it out into a complex 

 series of valleys and ridges till it ceased to rise farther. The 

 denudation continuing, the whole mass became worn away into 

 lowland plains and valleys. Then, after a long period of 

 quiescence, subsidence began, and as the land sank beneath the 

 water new deposits were laid down over it. Sometimes re- 

 peated elevations and depressions of small extent occurred; 

 while at very long intervals there was great and long-continued 

 subsidence, and, while deeply buried under newer strata, the 

 older masses were subjected to intense subterranean heat and 

 compression, which altered their texture, and often crumpled 

 and folded them up in the strangest manner conceivable. 

 Then, perhaps, a long period of elevation brought them up and 

 up, till they were many thousand feet above sea-level ; and, 

 when the superficial covering of newer beds had been all re- 

 moved by denudation, the folded strata were themselves ex- 

 posed to further denudation, and all the strange peaks and 

 ravines and rushing cataracts of alpine mountains became re- 

 vealed to us. 



1 This sketch of the internal structure of the earth, as affecting elevation 

 and depression of its surface, is fully discussed in INIr. 0. Fisher's Physics 

 of the Earth's Crust, a popular abstract of which is given in my Studies 

 Scientific and Social, vol. i. chap. iii. 



