226 A. Tylor on Changes of the Sea-Level. 
changes of the level of the sea-bottom, or that of the sea itself, 
or of both conjointly. 
Great caution must also be requisite in judging of the time 
occupied in the formation of the older rocks from their mineral 
character, as the following description of passing events will also 
apply to periods that are long gone by. 
Mr. Austen relates in one of his papers, that “ with a continued 
gale from the west large areas of the dredging-grounds on the 
French coast became at times completely covered up by beds of 
fine marly sand, such as occurs in the offing, and which becomes 
so hard that the dredge and sounding-lead make no impression 
upon it: with the return of the sea to its usual condition, a few 
tides suffice to remove these accumulations.’ 
Mr. Deane, the submarine surveyor, also reported to the Insti 
tution of Civil Engineers, that the turn of the tide is felt as soon 
near the sea-bottom at a depth of 120 feet as it is at the surface ; 
and he represents that the loose materials covering the Shambles 
Rocks are moved backwards and forwards with every tide. 
With these facts before us, what criterion can there be (even 
by estimating the sources of the detritus) for arriving at the minr 
mum or maximum rate at which sands and marls become pel- 
manent additions to the sea-bed? For the materials may present 
all the appearances of hasty accumulation, and yet the interval of 
time between the deposit of two strata of sand now contiguous 
may have been occupied by countless temporary deposits, a 
quickly brought and as quickly removed by the tide, and leaving 
no trace whatever of their existence. For the same reasons, W® 
removed in the next. It is therefore possible that many such 
movements may have occurred, and that the delta of the Missis- 
t 
ceding part of the paper the conclusion was arrived at, wee 
taking an extreme view of the rapidity with which the mater! id 
k cou 
level 
here must be, however, many rivers which are oD 
afford very small supplies of mud to any alluvial formations, 
7 Quart. Journ. Geol. Soc., vol. vi, p. 79. theomi 
t It is hoped that in the course of a few years enough data will be fo 1 ovieal 
to determine more nearly the importance of this variation of level in 4 
point of view, 
