existing Physical Causes during stated Periods of Time. 225 
that a supply of 10 cubic feet of sand or mud is obtained from 
each foot of frontage of any coast-line, and distributed 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 quatitity of material; but although tides and currents are — 
at work removing the sea-bed in one place and forming sediment- 
ary 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 lowest, 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 
_The limited supply of detritus derived from cliffs, and the wide 
distribution of that from rivers, renders it difficult to imagine any 
very extensive tract of sea-bottom where the rate of deposit de- 
tived exclusively from new materials should many times excee 
the average. Even on areas where extreme cases of denudation 
and deposition occurred (in periods when the sea-bottom was 
unaffected by movements, subsidence and elevation), there would 
many parts where the condition of depth would remain un- 
altered, because on them the rise in the sea-level would compen- 
For 
ormation 
fontaining throughout the remains of the same species of Mol- 
the equal depth of water indicated by the organic remains had 
been preserved during the formation of the deposit by means of 
* The effect h of the ocean would be of little 
i f th s ihe general depth o : : 
Mportance in & iediaglin Sakae of ve. except for an extende period of time, such 
48 Must have elapsed during the construction of a great serial group of strata. 
Stcoxp Serres, Vol. XVIII, No. 53—Sept, 1854. " 
