existing Physical Causes during stated Periods of Time. 259 



Section of the Bar of the Mississippi. 



The Bar. 



Stratum of fresh water, 7 feet deep, run- 

 ning per hour from 2 to 2^ miles, and 

 passing over the Bar without losing its 

 velocity. 



2. Stratum of salt water, 1 foot deep, running 



at the same speed as No. 1. 



3. Ditto, running at 1 mile per hour. 



4. Ditto, supposed to be stationary. 



5. Ditto, flowing towards the Bar, 



Fig. 2. Theoretical Diagram of four successive Bars, with their 

 ^ accompanying fluviatile deposits. j^j^^^. 



1} 



Fluvia- 

 tile for- 

 mations. 



a, b, c, d, 1st, 2nd, 3rd, and 4th Bars of marine formation. 



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 forma- 

 tions familiar to geological observers. 



But the subsidence and elevation of the crust of the earth 

 would be accompanied by alterations of the area of the sea-bed ; 

 and the frequency of such movements would therefore furnish 

 additional reasons for not considering the sea-level permanent 

 for the lengthened periods requisite for the accumulation of 

 sedimentary deposits of any magnitude. 



In the Third Part of this paper an attempt is made to direct 

 attention to the difficulty of finding any test by which to distin- 

 guish 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 sta- 

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

 occurred of sufficient importance to cause the temoval of the 

 MoUusca 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 



* " In formations from a few hundred to a thousand feet and upwards in 

 thickness, the whole of which does actually belong to the same geologic^,! 



