Ransome.] 



The Great Valley. 



419 



accounted for by a squeezing out of some of the lower unconsoli- 

 dated layers by the pressure of the superincumbent mass, and indi- 

 cates that the decomposition of vegetable matter, and molecular 

 condensation, may be responsible for a part of the superficial sub- 

 sidence. On the whole, however, it seems to be a well-established 

 fact that most of the great deltas of the world are built up on sub- 

 siding areas; and, it may be added, necessarily so, for a rising area 

 would obviously tend to divert the seat of deposition to another 

 point, while an area which was neither rising nor sinking would 

 soon be filled up to the baselevel, and the river would then carry 

 its load further on to a more commodious place of deposition. 

 Viewed from this aspect, there is nothing in the subsidence 

 of a delta that requires the hypothesis of isostasy. The drainage 

 of a great continental area will converge toward that portion of the 

 adjacent sea-coast which is subsiding, and will continue to deposit 

 its burden of sediment at that point as long as the movement con- 

 tinues. Moreover, deltas are conspicuously regions of superabun- 

 dant sediment. Not only does the river make good by its deposits 

 the subsidence of the earth's crust, but it carries the surplus further 

 on, and pushes its delta seaward. Thus 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 of pari passu local sinking under 

 load, but which indicates a subsidence of more general extent, and 

 independent of such local loading, 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 sub- 

 sidence to the south, it has covered the floor of the valley with 

 relatively thin deposits only, and advanced persistently out into 

 the Gulf. 



Such subsidences of sediment-laden areas as we have been con- 

 sidering are generally made manifest by an encroachment of the 

 sea upon adjacent portions of the land which are not receiving 

 sediment. In some cases this subsidence appears to be in excess of 

 4 



