1854.] 



ON THE CHANGES OF THE SEA-LEVEL. 



57 



($7-62) per evening, or 1-9 cents per man. The economy is 

 considerable, and the work can be done -without danger and 

 with a regularity which cannot be obtained by any other 

 means. 



The Perpetual Secretary remarked, that electro-lighting 

 which could be very cheaply established on ship-board, and 

 which is not, like other systems of lighting, liable to be ex- 

 tingui.shed during a storm, would be very advantageous for 

 preventing those collisions by night which are so frequent, and 

 generally so disastrous, and to which attention has been called 

 by a recent event. — Vomptes Rcndus. 



On Changes of the Sea-Level effected by existing Physical 

 Causes during stated periods of time. 



BY ALFRED TYLOR, F.O.S.* 



Introduction. 

 The First Part of the ensuing paper is occupied with the 

 details of the probable amount of the solid matter annually 

 brought into the ocean by rivers and other agents, in suspen- 

 sion and solution ; and the conclusion is arrived at, that the 

 quantity of detritus thus distributed on the sea-bottom would 

 displace enough water to cause an elevation of the ocean-level 

 to the extent of at least 3 inches in 10,000 years. 



In the Second Part an endeavour is made to compute the 

 number of such periods of 10,000 years that must have elapsed 

 during the accumulation of the immense mass of recent fresh- 

 water strata said to exist in the valley of the Mississippi. 



The calculation as to the latter is made from the data 

 collected by observers in America, of the extent of the deposit 

 in question ; and it is here supposed, first, that in former 

 periods the same quantity of mud as at present has been 

 annually carried into the Gulf of Mexico ; and secondly, that 

 the amount of sediment deposited on the delta and plains of 

 the Mississippi does not exceed one-tenth part of the solid 

 material which has been carried out (suspended in the water 

 of the river) into distant parts of the Gulf of Mexico, or into 

 the Atlantic Ocean itself. 



From recent accounts by Mr. C. Ellet, of the United States, 

 it appears that a column of fresh water, 1 \ mile wide and about 

 7 feet deep, is constantly entering the Gulf of Mexico at a 

 speed of 2 to 2 J miles per hour, and floats on the surface of a 

 stratum of salt water, to which it partially communicates its 

 own velocity. And below this a stratum of sea-water is found 

 to be flowing in an opposite direction to that of the two strata 

 of fresh and salt water above it. 



From the data submitted, it would appear that the accumu- 

 lation of the alluvial deposit of the Jlississippi must have 

 occupied a great number of periods, during each of which an 

 elevation of the sea-level of o inches may have occurred. 



* From the Philosophical Magazine for April, 18.5.'j. 



j " In formationsfrom a few hundred to a thousand feet and upwards 

 in thickness, the whole of which does actually belong to the same 

 geological age and is therefore characterized by the same fossils, most 

 curious and important results may be sometimes deduced if the position 

 or relative heights at which the groups of fossils are imbedded be 

 noted ; and this is a point usually neglected. For, thanks to the re- 

 searches of Professor E. Forbes, the depth of water imder which a 

 coUcction of shells lived can now be approximately told ; and thus the 

 2 



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 construc- 

 tion of a recent deposit like that in the valley of the Mississippi, 

 which may be called small and local compared with those older 

 formations 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 difiiculty of finding any test by which to dis- 

 tinguish 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 station- 

 ary. In either ease no change of depth of water may have 

 occurred of sufiicient importance to cause the removal of the 

 Mollusca inhabiting the locality, and therefore the discovery of //ie 

 same species of orcjanic remains from top to hottom of a thick 

 deposit is not an alosolute proof (as has been supposed"]") that 

 gradual subsidence has occurred during that particular forma- 

 tion ; because the condition of equal depth of water during 

 any deposit might be produced either bj' subsidence of the sea- 

 bottom or elevation of the sea-level, or by both conjointly. 



In discussing these questions, the writer has not assumed 

 that during gradual subsidences or gradual elevations, greater 

 denudations or depositions would occur than when the level of 

 the land and sea-bottom was stationary; because it is not certain, 

 either that during such gentle oscillations the forces that would 

 produce denudation are sensibly diminished or increased, or 

 that the rocks which are brought within the reach of denuding 

 forces are necessarily more easily worn away than those which 

 were previously exposed to the same influences. 



P^VRT I. 



It has long been acknowledged that the quantity of detritus 

 annually carried into the ocean from various sources must 

 displace an equal volume of water, and thus tend to raise the 

 ■ level of the sea. Many j-ears since it was estimated by an 

 Italian that this change might amount to one foot in a thousand 

 years. The general opinion on this subject has been, that the 

 effects produced by the present supplies of detritus would be 

 too minute to be perceptible, and on geological enquiries the 

 ocean-level has been considered as permanent for all practical 

 purposes. I I here propose to offer the evidence of present 

 denudation in certain countries where careful observations 

 have been made, in order to show, that if .such rapid dcstiuc- 

 tion of land occurs in rao-st localities, then the operation of 

 present physical causes mu.st be amply sufficient to eflfcct a 



movement of the crust of the earth, while the strata including the shells 

 were accumulating, can be inferred. 



" For instance, if the bottom of a cliff, say 800 feet in height, a set 

 of shells are buried which must have lived under water only 50 or 

 100 feet in depth, it is clear that the bottom of the sea mtlst have sunk 

 to have allowed of the deposition of the 700 feet of superincumbent 

 submarine strata ; subsequently the whole 800 feet must have been 

 upraised." (Darwin.) 



% JIanfrodi. See Lyell's Principles, edit. 1850, p. 270 and 542. 



