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 

 depend more upon the elevation of Central America 

 and the Isthmus of Panama and the inter tropical 

 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 

 was much warmer or colder than it is now, it w r ould 

 have a tendency to retain that higher or lower tem 

 perature for a succession of geological epochs. . . . 

 The slowness of climatical change here alluded to 

 would arise from the great depth of the sea as com 

 pared with the height of the land, and the con 

 sequent lapse of time required to alter the position 

 of continents and great oceanic basins. . . . The 

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

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

 of vertical movements equally 1,000 feet in both 

 directions, upwards and downwards, is to cause a 

 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. 

 But movements of equal amount would have no 

 tendency to produce a sensible alteration in the 

 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. 55 1 

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



