existing Physical Causes during stated Periods of Time. 279 



than the mean rate, the variation in the depth of water would be 

 proportionately less than if the sea-level had been permanent. 



The limited supply of detritus derived from cliffs, and the 

 wide distribution of that from rivers, renders it difficult to ima- 

 gine any very extensive tract of sea-bottom where the rate of 

 deposit derived exclusively from new materials should many times 

 exceed the average. Even on areas where extreme cases of de- 

 nudation and deposition occurred (in periods when the sea-bottom 

 was unaffected by movements, subsidence and elevation), there 

 would be many parts where the condition of depth would remain 

 unaltered, because on them the rise in the sea-level would com- 

 pensate the addition to the sea-bottom. Also if, in periods that 

 are past, the supplies of detritus from rivers and cliffs were many 

 times greater than at present, they must have caused propor- 

 tionately greater fluctuation of the sea-level, and therefore under 

 such circumstances there would also be parts of the oceanic area 

 receiving deposit at the same rate that the sea was rising. There 

 would thus have been opportunities for the accumulation of 

 sedimentary rocks without any change taking place in the depth 

 of the water they were formed in, during the intervals when the 

 sea-bottom was undisturbed by subsidences and elevations. For 

 these reasons, in examining the section of a marine formation 

 containing throughout the remains of the same species of Mol- 

 lusca, it would require independent evidence to determine whether 

 the equal depth of water indicated by the organic remains had 

 been preserved during the formation of the deposit by means of 

 changes of the level of the sea-bottom, or that of the sea itself, 

 or of both conjointly. 



Great caution must also be requisite in judging of the time 

 occupied in the formation of the older rocks from their mineral 

 character, as the following description of passing events will also 

 apply to periods that are long gone by. 



Mr. Austen relates in one of his papers, that " with a con- 

 tinued gale from the west large areas of the dredging-grounds 

 on the French coast became at times completely covered up by 

 beds of fine marly sand, such as occurs in the offing, and which 

 becomes so hard that the dredge and sounding-lead make no 

 impression upon it ; with the return of the sea to its usual con- 

 dition, a few tides suffice to remove these accumulations*.^^ 



Mr. Deane, the submarine surveyor, also reported to the 

 Institution of Civil Engineers, that the turn of the tide is felt as 

 soon near the sea-bottom at a depth of 120 feet as it is at the 

 surface ; and he represents that the loose materials covering the 

 Shambles Rocks are moved backwards and forwards with every 

 tide. 



* Quart. Journ. Geol. Soc. vol. vi. p. 79. 



