CHAP, x.] CONTINUITY OF TEE 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 cre 

 taceous rocks. He 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 w r ould, 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." 



Erom 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. 



