48 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 15, 1852. 



observers in America, as to the extent of the deposits in question : 

 and it is supposed, first, that in former periods the same quantity of 

 mud, as at present, has been annually carried into the Gulf of 

 Mexico ; and secondly, that the amount of sediment deposited in the 

 delta and plains of the Mississippi does not exceed one-tenth part of 

 the solid material which has been carried out (suspended in the water 

 of the river) into distant parts of the Gulf of Mexico, or into the 

 Atlantic itself. From the data submitted, it would appear that the 

 accumulation of the allmdal deposits of the Mississippi must have 

 occupied a great number of periods, during each of which a mean 

 elevation of the sea-level to the amount of at least three inches may 

 have occurred ; and that the removal of detritus from the 1,100,000 

 sq. miles of North America drained by the Mississippi would (if annu- 

 ally carried on at the same rate as at present) reduce the mean sur- 

 face-level of that district one foot in 9000 years ; whilst the Ganges 

 would produce the same effect on the area it drains in 1/91 years. 



The general conclusion arrived at is, that the sea-level cannot be 

 considered as stationary for practical geological purposes, since the 

 operation of present physical causes would produce a considerable 

 change in its height, even during the construction of a recent deposit 

 like that in the valley of the Mississippi, which may be called small 

 and local compared with those older formations familiar to geological 

 observers. 



As the subsidences and elevations of the crust of the earth would 

 be accompanied by alterations of the area of the sea-bed, the fre- 

 quency of such movements would necessarily furnish additional rea- 

 sons for not considering the sea-level permanent for the lengthened 

 periods requisite for the accumulation of sedimentary deposits of any 

 magnitude. 



Lastly, the author directs attention to the difl&culty of finding any 

 test by which to distinguish strata gradually accumulated during a 

 long- continued upward movement of the sea-level, from those strata 

 formed on a sea-bottom slowly subsiding while the ocean-level was 

 stationary. In either case no change of depth of water may have 

 occurred of sufficient importance to cause the removal of the Mollusca 

 inhabiting the locality, and therefore the discovery of the same species 

 of organic remains from top to bottom of a thick deposit is not an 

 absolute proof (as has been supposed) that gradual subsidence has 

 occurred during the formation of that particular deposit, for the con- 

 dition of equal depth of water during any deposit might be produced 

 either by subsidence of the sea-bottom, or elevation of the sea-level, 

 or by both conjointly. In discussing these questions, the author has 

 not assumed that during gradual subsidences or gradual elevations, 

 greater denudations or depositions would occur, than when the level 

 of the land and sea-bottom was stationary ; because it is not certain 

 either that during such gentle oscillations the forces that would pro- 

 duce denudation are sensibly increased or diminished, or that the 

 rocks which are brought within the reach of denuding forces are 

 necessarily more easily worn away than those which were previously 

 exposed to the same influences. Even if, in ancient periods, the rate 



