476 THE DEPTHS OF THE SEA. [CHAP. x. 



According to this view, the reduction of the tempera- 

 ture, the cause of the break in the fauna, would j 

 depend more upon the elevation of Central America | 

 and the Isthmus of Panama and the intertropical | 

 eastern coast of the continent of Asia, than even 

 upon the depression of the northern barrier and the | 

 throwing open of the Arctic basin. 



" If at any former period the climate of the globe j 

 was much warmer or colder than it is now 7 , it would j 

 have a tendency to retain that higher or lower tem- 

 perature for a succession of geological epochs. . . . I 

 The slowness of climatical change here alluded to j 

 would arise from the great depth of the sea as com- 

 pared with the height of the land, and the con- j 

 sequent lapse of time required to alter the position 

 of continents and great oceanic basins. . . . The j 

 mean height of the land is only 1,000 feet, the j 

 depth of the sea 15,000 feet. The effect, therefore, I 

 of vertical movements equally 1,000 feet in both ! 

 directions, upwards and downwards, is to cause a j 

 vast transposition of land and sea in those areas \ 

 which are now continental, and adjoining to which \ 

 there is much sea not exceeding 1,000 feet in depth, j 

 But movements of equal amount would have no \ 

 tendency to produce a sensible alteration in the j 

 Atlantic or Pacific oceans, or to cause the oceanic or 

 continental areas to change places. Depressions of 

 1,000 feet would submerge large areas of existing 

 land; but fifteen times as much movement would 

 be required to convert such land into an ocean of 

 average depth, or to cause an ocean three miles deep 

 to replace any one of the existing continents." l 

 1 Lyell, Principles of Geology, 1867. Pp. 265-6. 



