1854.] 



ON THE CHANGES OF THE SEA-LEVEL. 



59 



the ocean by tlie continued operations of rivers and breakers * 

 In speaking of the elevation of the sea-level, I only refer to the 

 intervals between those movements of the land which might 

 neutralize in an instant all that had been effected by the 

 operation of rivers for immense periods of time. 



It would add very much to the interest of this inquiry if 

 any proof could be brought forward of a recent gradual 

 upward movement of the sea-level. This would, however, be 

 difficult to observe, ■)• on account of the rise in the water con- 

 cealing the evidence of its former level, e.^cept just at the 

 mouths of rivers, where the deposits of fluviatile alluvium 

 might raise the land from time to time and keep it always 

 above the rising waters. 



The deposits situated at a few such localities have been 

 described by the best observers, and I hope to show that in 

 several cases there are appearances which might be partly 

 explained by changes of the sea-level, but that a much greater 

 number of cases and more certain evidence would be needed 

 before such an event could be satisfactorily proved. I propose 

 to make some remarks upon this point, after having submitted 

 the evidence which has induced me to believe that the supply 

 of detritus under present physical conditions is sufficient to 

 raise the ocean level 3 or 4 inches in 10,000 years, provided 

 no subsidence or elevation disturbed the result. 



To this subject I now proceed. Sir Charles Lyell's published 

 statements of the quantity of mud annually carried down by 

 the Mississippi and Ganges appear to have been made with so 

 much care, that they may be a better guide to the general rate 

 of removal of .soil by rivers than information obtained from a 

 greater number of smaller rivers, which of course are more likely 

 to be- influenced by local circumstances. Eleven hundred 

 thousand square miles of land are drained by the Mississippi,! 

 which annually discharges a quantity of water equal in volume 

 to 4 inches of rain or about one tenth of the total rain-fall over 

 this entire surface, which forms one-fifth part of North Ame- 

 rica.§ From the moan of a great number of observations, the 

 average quantity of alluvium suspended in the water appeal's 

 to be 1 part in 3000. Consequently, as the water annually 

 drawn off would cover an area of eleven hundred thousand 

 square miles to the depth of four inches, the quantity of mud 

 removed in the water, as measured at or near the mouth of the 

 river) would cover the same extensive surface to the depth of 

 l-3000th part of four inches, or to the depth of l-9000th part 

 of a foot. Or, in other words, the Mississippi at its present 

 rate would occupy 9000 years in carrying away detritus before 

 the mean surface level of one-fifth ])art of North America 

 would be reduced one foot. 



The Ganges discharges into the Indian Ocean a supply of 

 water equal to about six inches of rain on 400,000 square 

 miles, or a much greater volume of water than the Mississippi 

 pours into the Gulf of Mexico, taking into consideration the 

 difference in size of the countries they drain. 



The alluvium suspended in the waters of the Ganges is as 1 

 to 858 by weight ; consequently the detrital matter removed 

 in suspension by the water in one year would cover the land 

 from which it is derived to the depth of 1-1751 of afoot; that 

 is to say, the Ganges might pour out muddy water at its pre- 

 sent rate for 1751 years before the mean level of 400,000 , 

 square miles would be reduced one foot in height. The great 

 elevation of the Himalaya range, or possibly a greater rain-fall, 

 may probably occasion the difference between the rates of 

 denudation indicated by the Ganges and the Mississippi. As 

 there arc also parts of the earth's surface drained by rivers 

 flowing into lakes and inland seas, and other tracts are entirely 

 without rain, I propose to estimate (as before mentioned) that 

 only half the land contributes detritus in suspension to rivers 

 flowing directly into the sea.|| If this area be annually reduced 

 in level at the same rate, as the district through which the 

 Mississippi flows, then the mean level of the land on the globe, 

 would be reduced 3 ieet in 54,000 years, and consequently the 

 level of the ocean raised 1 foot in the same period by means of 

 the detritus suspended in river-water poured into the ocean. ^ 



But in addition to the sediment carried down by means of 

 rivers, we have also to take into consideration the amount of 

 debris washed into the sea from cliffs during so long a period 

 as that mentioned. It is difiicult, however, to form any esti- 

 mate of what this would annually amount to, for old maps and 

 charts are hardly accurate enough to represent the waste of 

 clifis by breaker-action even within the last 100 years. Capt. 

 Washington has, however, published a report** which gives an 

 account of the encroachment of the sea at intervals on one part 

 of the Sufiblk coast. This will give a general idea of the con- 

 tribution of detritus that may be obtained from some points of 

 a coast-line. The following statements are collected from 

 Capt. Washington's Keport on Harwich Harbor in 1844. 



The cliff on the western side of the harbor is about 1 mile 

 long and 40 feet high, and the encroachment of the sea appears 

 to have been at the rate of 1 foot per annum between the years 

 1709 and 1756, so that the annual supply of detritus was 

 equal to 40 cubic feet for each foot of frontage. Between 

 1756 and 1804 the advance increased to nearly 2 feet per 

 annum ; so that the annual removal of cliff amounted to nearly 

 80 cubic feet for each foot of frontage. 



Between 1804 and 1844 the encroachment of the sea 

 averaged 10 feet per annum, and the annual removal of detritus 

 must have amounted to 400 cubic feet for each foot of frontage. 

 It was during this latter period that exten.sive dredging for 

 cement stone took place at the base of the cliff. 



On the eastern side of the harbor events of an opposite char- 

 acter have occurred, for Landguard Point has gained 50 feet 

 per annum in length during the last 30 j'oars. The addition 

 thus made to the laud, and to the " littoral zone," prcsent.s an 

 interesting example of the rapid accumulation of a local de- 

 posit under favourable circumstances. From the appearance 



* "It is not necessary that the present land should be worn away 

 and wasted exactly in proportion as now land shall appear ; or con- 

 versely, that an equal proportion of new land should be produced as 

 the old is made to disappear." (Ilutton's Theory of the Earth, 1705, 

 vol. i, p. 190.) 



f See Darwin, Coi'.al Reefs, &c. edit. 1851, p. 95. 



J See art. Mississippi, Penny Cyclopoedia, vol. xxv, p. 277. 



? The total rain-fall of the Uuitcd States is 39 inches between 24J° 

 and 45° N. lat. (Bcrgluius and .Tohnston.) 



II The proportion of land withoutrain is about l-1200th of the whole. 

 Keith and .Johnston say that nearly one- half the drainage-water of 

 Europe and Asia falls into tl}e Black and Caspian Seas. The propor- 

 tion for Africa and America is not known. 



^ It Is not improbable that the solvent powers of vain and river-water 

 arc as important agents in the removal of land as the agency above 

 mentioned. Definite calculations on lliis subject remain to be made. 



** Tidal Harbors' Commission, First Report of 1845. 



