ON CHANGES OF THE SEA LEVEL.. 



[1854 



of the sea-bottom.* Even wliere deposits are taking place 

 mucli faster 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 im- 

 ao-ine 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 denudation 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 ojj them the rise in the 

 sea-level would compensate the addition to the sea-bottom. 

 Also if, in periods that are past, the supplies of detritus from 

 rivers and cliifs were many times greater than at present, they 

 must have caused proportionately greater fluctuation of the 

 sea-level, and therefore tinder such circumstances there would 

 also be parts of the oceanic area receiving deposits at the same 

 rate that the sea was rising. There would thus have been op- 

 portunities for the accumulation of sedimentary rocks without 

 any change taking place in the depth of the water they were 

 formed in, duringthe intervals when the sea-bottom was undis- 

 turbed by subsidences and elevations. For these reasons, in 

 examining the section of a marine formation containing 

 throughout the remains of the same species of Mollusca, 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 condition, a few tides suffice to remove these accumula- 

 tions."t 



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

 stitution 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 Eocks are moved backwards and forwards with 

 every tide. 



With these facts before us, what criterion can there be 

 (even by estimating the sources of the detritus) for arriving at 

 the minimum or maximum rate at which sands and marls be- 

 come permanent additions to the sea-bed ? For the materials 

 may present all the appearances of hasty accumulation, and yet 

 the interval of time between the deposit of two strata of sand 



* The effect of these causes on the general depth of the ocean would 

 be of little importance in a geological point of yiew, except for an ex- 

 tended period of time, such as must have elapsed during the construction 

 of a great serial group of strata. 



f Quart. Journ. Geol. Soc, vol. vi. p. 79. 



now contiguous may have been occupied by countless tem- 

 porary deposits, as quickly brought and as quickly removed by 

 tide, and leaving no trace whatever of their existence. For the 

 same reasons we cannot be certain that in the valley of the 

 Mississippi we have an unbroken sequence of fluviatile strata, 

 in which the accumulations of one century form the base for 

 those of the next, from the bottom to the top of the series ; be- 

 cause there, as in marine formations, the deposits of the one 

 period may have entirely been removed in the next. It is 

 therefore possible that many such movements may have occured 

 and that the delta of the Jlississippi may have occupied a 

 longer period of time for its formation than could be computed 

 from any data remaining. In the preceding part of the paper 

 the conclusion was arrived at, without taking an extreme view 

 of the rapidity with which the materials may have been col- 

 lected for its deposition, that the work could not have been 

 completed within a period for which the sea-level could be 

 considered permanent.'}" 



There must be, however, many rivers which are only able to 

 afford very small supplies of mud to any alluvial formations, 

 either from deriving their water from lakes or from countries 

 with a very small rain-fall. During a period when the gradual 

 elevation of the sea-level was not counteracted by the effects of 

 more powerful causes, there would be conditions near the 

 mouths of some rivers of this kind for the surface of their 

 plains to be gradually elevated by the operation of winter floods 

 at a rate somewhat similar to that of the sea-level. In this 

 manner purely fluviatile deposits might be formed in the 

 neighborhood of the ocean, occupying positions similar to that 

 represented in the lower part of the longitudinal section of the 

 IMississippi, without the necessity of supposing any subsidence 

 of the land. In the upper portions of such rivers, the periodi- 

 cal floods, assisted by the accumulation of teiTestrial remains 

 in the adjoining plains, would add stratum after stratum dur- 

 ing periods when the surface of the country was unaffected by 

 subterranean movements. It is probable that the rate of de- 

 posit might be accelerated in periods of subsidence ; but the 

 manner in which rivers form plains along their course in all 

 countries under ordinary conditions, when no subsidence or 

 elevation is occurring, was traced by Hutton. 



Even if, in ancient periods, the rate of denudation was 

 greater than at present, and the supplies of detritus to rivers 

 more extensive, the fluctuations of the sea-level and the eleva- 

 tion of beds and plains of rivers would have been proportion- 

 ately greater. There would, therefore, still have existed some 

 localities where the rate of the formation of alluvial plains near 

 the sea kept pace with the elevation of the waters ; so that, as 

 at the present time, conditions would have existed for the ac- 

 cumulation of fluviatile strata containing terrestrial remains 

 without any subsidence of the land. This is a subject, how- 

 ever, that must be further sttidied, more especiaJy when its 

 value is considered in relation to the great masses of fluviatile 

 strata cither of the Mississippi, the Ganges, the Nile, or the 

 Po. For the above reasons it would be difficult to determine, 

 when examining sections of thick fluviatile strata, whether 

 these accumulations of detrital matter had been formed during 

 subsidence of the land, or during the gradual elevation of the 

 level of rivers and seas, arising from the continual operation of 

 ordinary physical causes. 



f It is hoped that in the course of a few years enough data will be 

 forthcoming to determine more nearly the importance of this variation 

 Of level in a geological point of view. 



