cHaP. X.] CONTINUITY OF THE CHALK. 475 
much higher and more equable temperature to the 
bottom ; and there is every reason to believe that such 
a land barrier did exist to the north of the great 
Atlantic basin, and continuous with the belt of 
northern land on which there is no deposition of ere- 
taceous rocks. Jle says that ‘“‘if such a land barrier 
existed at the period of the chalk, and that barrier 
was submerged during the earlier part of the tertiary 
period, it would, taken in conjunction with the very 
different conditions of depth under which the chalk 
and lower tertiaries were found, go far to account for 
the great break in the fauna of the two periods.” 
From the information we have as to the depths 
in the South Atlantic and the North Pacific, there 
seems to be no reason, however, to suppose that a 
barrier has recently existed shutting off the polar 
sea of the southern hemisphere; and I confess I 
cannot quite see how the result suggested by Mr. 
Prestwich could follow, without taking into account 
another condition of whose existence we seem to 
have evidence. A band of cretaceous rocks has been 
shown to extend round the world a little to the 
north of the equator wherever we have dry land; 
and it has likewise been shown, from considera- 
tions of depth, that this chalk band probably ex- 
tended also across our great ocean basins. At that 
time, then, it seems that no continent ranging from 
north to south interrupted the drift of the equatorial 
current, deflecting the heated equatorial water to 
north and south and inducing a return indraught 
of polar water. ‘This would undoubtedly remove 
one great cause, if not the sole cause, of the present 
low temperature of deep water between the tropics. 
