260 Mr. A. Tylor on Changes of the Sea-level effected by 



gradual subsidence has occurred during that particular formation ; 

 because the condition of equal depth of water during any deposit 

 might be produced either by 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 denu- 

 dations 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 pro- 

 duce 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 pre- 

 viously exposed to the same influences. 



Part I. 



It has long been acknowledged that the quantity of detritus 

 annually carried into the ocean from various sources must dis- 

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

 of the sea. Many years 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 inquiries the ocean-level has 

 been considered as permanent for all practical purposes*. 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 destruction of land occurs in most loca- 

 lities, then the operation of present physical causes must be 

 amply sufficient to effect a perceptible alteration in the sea-level 

 in a moderate space of time. 



The mere consideration of the number of cubic feet of detritus 

 annually removed from any tract of land by its rivers, does not 

 produce so striking an impression upon the mind as the state- 

 ment of how much the mean surface level of the district in ques- 



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 researches of Prof. E. Forbes, 

 the depth of water under which a collection of shells lived can now be 

 approximately told ; and thus the movement of the crust of the earth, 

 whilst the strata including the shells were accumulating, can be inferred. 



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

 shells are buried which must have lived under water only 50 or 1 00 feet in 

 depth, it is clear that the bottom of the sea must have sunk to have allowed 

 of the deposition of the 700 feet of superincumbent submarine strata ; sub-, 

 sequently the whole 800 feet must have been upraised." (Darwin.) 



♦ Manfredi. See Lyell's Principles, edit. 1850, p. 270 and 542. 



