456 STUDIES FOR STUDENTS 



covered continually with sufficient vegetation to leach out by its decay 

 the iron from the deposited sediments, but not standing in water, 

 and therefore finally destroyed by oxidation. 



On the seaward side the subsidence brings about a transgression 

 of the sea with considerable erosion of the forerunning transitional 

 littoral zone, or even of the underlying fluviatile and swamp forma- 

 tions, such as is sometimes observed to have occurred in the coal- 

 measures of Illinois, Kentucky, and Missouri. In order to observe 

 the process of land-reclamation by the river deltas from the sea, sup- 

 pose, to continue the example, that the subsidence has been sufficiently 

 rapid for the sea to gain 10,000 square miles from the land with an 

 average depth of 50 feet, at which time the subsidence ceases. 



The Ganges annually carries across its delta to the sea sufficient 

 sediment to cover one square mile 221 feet deep; the Mississippi 

 annually discharges into the Gulf of Mexico sufficient to cover one 

 square mile 268 feet deep. Applying these figures to the hypotheti- 

 cal case, and assuming that one-half of the discharged sediment goes 

 to make the fore-set beds by which the delta is built outward, it is 

 seen that the Ganges would completely reclaim this area in 4,524 

 years, the Mississippi in 3,730 years. These, however, are two of 

 the greatest rivers. But even if the carboniferous rivers discharging 

 across the region of the Appalachian coal-fields delivered but a tenth 

 part of the detritus borne to the sea by the Ganges and the Mississippi, 

 it is seen that the transgressive effect of the supposed subsidence 

 would be completely nullified in periods of 45,240 and 37,300 years. 

 During this period of quiet and of land extension, conditions for the 

 formation of coal would exist over much of the delta surface not 

 actually traversed by the rivers, the swamps by the deposit of organic 

 debris keeping pace to some extent with the distributaries raising 

 their beds as the delta advances, and thus tending to prevent the 

 wandering of the rivers across them. 



In the present connection it is desired to emphasize not only the 

 upbuilding but the outbuilding capacity of rivers, by which, if their 

 sources are in highlands, their deltas may rapidly push into and 

 fill up shallow epicontinental seas. Under such circumstances it is 

 largely a question of the rate of river deposition and the volume of 

 the sea to be filled. 



