278 Mr. A. Tylor on Changes of the Sea-Level effected by 



ments of the sea-bottom from time to time, but particularly 

 mentions that it was not necessary to suppose that the dry land 

 was equally extensive at all periods. Since the fluctuation in 

 the sea-level would be directly consequent upon the destruction 

 of land arising from . the operation of rain, the atmosphere, and 

 running water on its surface, such changes would be in harmony 

 with the spirit of the Huttonian theory. 



Part III. 



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

 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 supply, while other locali- 

 ties nearer the coast-line obtain a great deal more than the average. 

 In the interval between these places, where the rate of deposit 

 is extremely high, and those where it is extremely low, must lie 

 an extensive tract of sea-bottom, where the accumulation of 

 detritus does not much differ from the average rate, 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 supplies 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 distri- 

 buted 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 to 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 forming 

 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 low- 

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

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

 they may be situated, are particularly interesting, because 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 of the sea- 

 bottom*. Even where deposits are taking place much faster 



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

 of little importance in a geological point of view, except for an extended 

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

 serial group of strata. 



