280 Mr. A. Tylor on Changes of the Sea-Level. 



With these facts before us, what criterion can there be (even 

 by estimating the sources of the detritus) for arriving at the 

 minimum or maximum rate at which sands and marls become 

 permanent additions to the sea-bed ? For the materials may pre- 

 sent all the appearances of hasty accumiilation, and yet the interval 

 of time between the deposit of two strata of sand now contiguous 

 may have been occupied by countless temporary deposits, as 

 quickly brought and as quickly removed by the tide, and leaving 

 no trace whatever of their existence. For the same reasons, we 

 cannot be certain that in the valley of the Mississippi we have 

 an unbroken sequence of fluviatile strata, in which the accumu- 

 lations of one century form the base for those of the next, from 

 the bottom to the top of the series ; because there, as in marine 

 formations, the deposits of one period may have been entirely 

 removed in the next. It is therefore possible that many such 

 movements may have occurred, and that the delta of the Missis- 

 sippi may have occupied a longer period of time for its formation 

 than could be computed from any data remaining. In the pre- 

 ceding part of the paper the conclusion was arrived at, without 

 taking an extreme view of the rapidity with which the materials 

 may have been collected for its deposition, that the work could 

 not have been completed within a period for which the sea-level 

 could be considered permanent*. 



There must be, however, many rivers which are only able to 

 afford very small supplies of mud to any alluvial formations, 

 either from deriving their water from lakes or from countries 

 with a very small rain-fall. During a period when the gradual 

 elevation of the sea-level was not counteracted by the effects of 

 more powerful causes, there would be conditions near the mouths 

 of some rivers of this kind for the surface of their plains to be 

 gradually elevated by the operation of winter floods at a rate 

 somewhat similar to that of the sea-level. In this manner purely 

 fluviatile deposits might be formed in the neighbourhood of the 

 ocean, occupying positions similar to that represented in the 

 lower part of the longitudinal section of the Mississippi, without 

 the necessity of supposing any subsidence of the land. In the 

 upper portions of such rivers, the periodical floods, assisted by the 

 accumulation of terrestrial remains in the adjoining plains, would 

 add stratum after stratum during periods when the surface of 

 the country was unaffected by subterranean movements. It is 

 probable that the rate of deposit might be accelerated in periods 

 of subsidence ; but the manner in which rivers form plains along 



* It is hoped that in the course of a few years enough data will be forth- 

 coming to determine more nearly the importance of this variation of level 

 in a geological point of view. 



