1854.] 



ON CHANGES OF THE SEA LEVEL. 



But it is also necessary to estimate what proportion of the 

 total quantity of mud brought down by the river is carried 

 completely out to sea, compared to what is left either upon the 

 marine or fluviatile portion of the delta. 



Sir Charles Lyell has remarked, that the alluvium now re- 

 maining in the valley of the Mississippi can only represent a 

 fragment of what has passed into the Gulf of Mexico ; and this 

 can readily be believed when we reflect upon the depth and 

 breadth of the channel, and upon the short period of the year 

 that the stream would throw any large quantity of mud into the 

 plains even if there were no artificial banks. We must also 

 bear in mind that only the coarse mud could settle near the 

 shore, for the finer particles could not deposit except in very 

 deep water. For these reasons, even if the mud carried be- 

 yond the mouth of the river is only ten times the quantity left 

 behind on the fluviatile portion of the delta and plains of the 

 Mississippi, this amount of detritus could not be obtained with- 

 out the mean level of one-fifth part of North America being 

 reduced 100 feet by denudation affe';ted by the action of rain, 

 the atmosphere, and running water.* But Hutton (vol. ii, p. 

 401) remarks, in 1795, that wherever any stream carried ofi" 

 particles of soil in its waters at any period of the year, it might 

 be said that denundation was taking place in that country ; yet 

 he particularly observed that the waste of land was very un- 

 equal, being much more rapid in the elevated than in the more 

 level parts of any district. It is therefore possible that, dur- 

 ing the reduction of the mean surface-level of the land drained 

 by the Mississippi to the amount of 100 feet, some portions of 

 the area might be lowered many times that amount, while 

 other portions might suffer little, or be positively raised by the 

 superposition of alluvial deposits. We are, however, informed 

 by Sir Charles Lyell, that the Mississippi in one part of its 

 course cuts through ancient fluviatile beds evidently antecedent 

 to those recent deposits we have been considering. This for- 

 mation is also stated to contain the remains of species of plants 

 and animals now existing ; so that evidence is to be obtained 

 in this district of still greater denudations (by these results) 

 than those of which we have spoken, and which would produce 

 changes on the surface of the earth since the introduction of 

 the present fauna and flora of extent enough almost to realize 

 Hutton's vision of mountains wasted away by the action of 

 rain, the atmosphere, and running-water, and carried along 

 river-courses into the ocean. It is not necessary to take an 

 extreme view of this subject to gain the object we have in 

 view, which is to show that, during the time occupied by the 

 formation of the jMississippi delta, the sea-level might be per- 

 ceptibly raisodf by the agency of physical causes now in ope- 

 ration. 



The reasons for supposing that a rise of 3 inches in each 

 period of 10,000 years might occur, have been already discuss- 

 ed, and it only remains to state that, at the present rate of 

 denudation, it would rc(|uire five such periods to produce the 

 quantity of detritus said to exist in the valley of the Missis- 

 sippi ; while it would rec|uirc fifty such periods to produce the 

 requsite quantity of alluvium on tlie supposition that only one- 

 tenth of the mud in trausifit through the river was appropri- 

 ated for the accumulation of its alluvial plains and deltas. Un- 



* The (lata for calculating the annual quantity of detritus carried 

 over the river's banks, in relation witli that carried down to tlic sea, are 

 vei-y imperfect. Fm-ther iuformnticu on this suhjcct is much ueeUed 



f This change of level may amount, under certain circumstances, to 

 a great extent, but at the lowest calculation would be 15 feet. 



der these circumstances it appears a legitimate conclusion, that 

 the level of the sea cannot be considered permanent for all 

 practical pui-poses when it may be shown that it might be dis- 

 turbed by the operation of present causes during the period 

 occupied by the construction of a single geological formation. 

 Elevations and subsidencies of the land or sea-bottom would 

 also effect important changes in the height of the sea-level, 

 sometimes counteracting and at others adding to the eflects 

 produced by the continuous operation of rivers, &c. The ef- 

 fects produced by these important causes would be an additional 

 reason for not cosidering the sea-level permanent. 



It is hardly necessary to add, that the continual waste of the 

 earth's surface by the carrying of materials into the ocean by 

 rivers and breakers particularly attracted the attention of Hut- 

 ton. He considered* that this was counteracted by elevator}' 

 movements of the sea-bottom from time to time, but particu- 

 larlj' mentions that it was not necessary to suppose that the 

 dry land was equally extensive at all periods. Since the fluc- 

 tuation in the sea-level would be directly consequent upon the 

 destiiiction of land arising from the operation of rain, the at- 

 mosphere, and running water on its surface, such changes 

 would be in harmony with the spirit of the Hutton ian therory. 



Part III. 



The average thickness of the deposit formed on the sea-bot- 

 tom by the solid materials brought on to it from all sources 

 has been estimated in the preceding part of the paper at 3 

 inches in 10,000 years, producing an elevation of that amount 

 in the sea-level in the same period. Some portion of the 

 oceanic area may be supposed to receive no part of this sup- 

 ply, while other localities nearer the coast-line obtain a great 

 deal more than the average. In the inteiTal between these 

 places, where the rate of deposit is extremely high, and those 

 where it is extremely low, must lie on extensive tract of sea- 

 bottom, where the accumulation of detritus does not much 

 differ from the average r^te, which we have supposed to be 3 

 inches in 10,000 years. Such localities may be more extensive 

 near those parts of the ocean-bottom which receive no stipplies 

 of detritus whatever, but they must stretch up to the coast-line 

 in many places. For instance, if it is supposed that a supply 

 of 10 cubic feet of sand or mud is obtained from each foot of 

 frontage of any coast-line, and distributed between high-water 

 mark and 20 miles distant, it might raise the mean level of 

 that portion of sea-bottom 1 foot in 10,000 years. 



Rivers opening on the shore might also bring down a still 

 greater quantity of material ; but although tides and currents 

 are at work removing the sea-bed in one place and furuiing 

 sedimentary strata in others from the old and new materials, 

 there must everywhere be portions of every sea-bottom where 

 the rate of deposit is intermediate between the highest and 

 lowest, and may often not differ much from that of 3 inches in 

 10,000 years. These portions of the great oceanic area, 

 wherever they maj- be situated, are particularly interesting, be- 

 cause on them the accumulation of sedimentary deposit is 

 taking place without any change in the depth of water, and yet 

 without necessitating the supposition of gradual subsidence 



* These remarks of Hutton aio here introduced because ho takes an 

 entirely different view of this subject to that promulgated by Sir 

 Charles Lyell, who considers tliat there lias been always an excess of 

 subsidence. (See principles, 1850, p. 5-13.) 



