186 31 g Gee — Gulf of Mexico as a measure of Isostasy. 



from a many times larger degradation tract (figure 2). On 

 closer inspection the first impression is strengthened : The 

 northern half of the Gulf with the adjacent lands (of which 

 alone the geologic history has been clearly read) is a province 

 of simple structure, of limited and uniform continental move- 

 ments since the middle Cretaceous ; furthermore, it is this 

 northern half of the Gulf which receives the drainage from 

 the second largest degradation tract of the globe ; moreover, 

 one of the strongest oceanic currents of the globe — the main 

 Atlantic equatorial current — enters the Gulf through Yucatan 



channel and sweeps through the basin in such fashion as to con- 

 centrate the sediments upon a narrow zone skirting the northern 

 border of the basin; and finally the influence of sedimentation 

 is not confined to a single delta but is so distributed that the 

 rate of deposition is variable in different parts of the littoral 

 zone, though in simple and easily ascertained fashion. 



Unfortunately the Gulf coast has only recently been sur- 

 veyed with precision, and the surveys have not yet been re- 

 peated in such manner as to give quantitatively exact meas- 

 urements of land-subsidence or sea-encroachment ; but the 



