A76 THE DEPTHS OF THE SEA. [cHaP, xX. 
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 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. 
“Tf at any former period the climate of the globe 
was much warmer or colder than it is now, it would 
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.” ' 
1 Lyell, Principles of Geology, 1867. Pp. 265-6. 
