REVIEWS 731 



ular maybe noted: "The Mississippi River, during later geological 

 times, has pushed its delta from Cairo out to its present termination 

 in the Gulf of Mexico, an extension that is not compatible with the 

 idea oi pari passu local sinking under load, but which indicates a sub- 

 sidence of more general extent, and independent of such local load- 

 ing, inasmuch as the maximum effect has been, not near Cairo, at the 

 original point of loading, but to the south of it — probably in the deep 

 hollows of the present Gulf of Mexico. The river did not stop to 

 build up a thick mass of delta accumulations at Cairo, as it should 

 have done according to the theory of isostasy ; but, attracted onward 

 by the independent subsidence to the south it has covered the floor of 

 the vallev with relatively thin deposits only, and advanced persistently 

 out into the Gulf." 



Now the real reason that the Mississippi River did not build up a 

 thick mass of delta accumulations at the head of the present embay- 

 ment, "as it should have done according to the theory of isostasy" is 

 that Cairo is probably not onlv not " the original point of loading," 

 but the latter point is to the south of it, far to the south, in fact nearly 

 as far as New Orleans. This is at once apparent from a glance at the 

 history of the region. At the close of the Cretaceous or early in the 

 Tertiary the whole region was a vast graded surface, or peneplain, the 

 southern limit of which was as far south at least as central Louisiana. 

 The present Mississippi River was a comparative insignificant stream 

 with none of its present tributaries beyond the present city of St. 

 Louis. The peneplain is found on the east side of the embayment 

 dipping westward and on the west side dipping eastward under the 

 unconsolidated elastics of the embayment, thus forming a broad shal- 

 low svncline, as recently shown by Griswold. The warping which has 

 taken place since the beginning of this down-sinking along the line of 

 the Mississippi has allowed a relatively thin mantle of later sediments 

 to be deposited over a large area. While at various times the waters 

 of a shallow sea may have been extended over a considerable portion 

 of the eujbayment "the original point of loading" was doubtless not 

 so very far from where it should be, nor is the present point of maxi- 

 mum depression and maximum loading far from "where it should be 

 according to the theory of isostasy." This, it may be added intro- 

 duces factors that are not commonly taken into account in the con- 

 sideration of the genesis of larg-e deltas. 



