166 
FORMATION OF DELTAS. 
ved from the disintegration of the sohd portion of 
the earth's surface, and carried by means of run- 
ning water into lakes and seas. But it must not be 
supposed that all is deposited upon the shores ; on 
the contrary, by far the largest portion is carried 
out to sea, and, by means of currents, swept to a 
distance. For example, if we look at the German 
Ocean, we shall see how small a part is deposited 
on the shores of Europe. The mean depth of this 
ocean is about 31 fathoms, and it is traversed by sev- 
eral enormous banks, one of which, called the Dag- 
ger Bank, extends upward of 354 miles in length; 
another extends 110 miles; and a third 105; and 
the whole superficies of these enormous shoals is 
equal to one fifth of the whole area of the German 
Ocean, or one third the whole extent of England 
and Scotland. The average height of the banks 
measures about 78 feet, consisting of fine and 
coarse silicious sands, mixed with broken corals and 
shells,*^ 
All these materials are deposited in regular strata, 
as we see in a sandbank or the sides of a river; 
and we have only to suppose them hardened into 
stone, to have an exact counterpart of the sand- 
stones, slates, and other rocks which we daily see 
upon the land. The accumulations being succes- 
sive, and sometimes interrupted, must give to such 
deposites the character of stratification. We thus 
learn how rocks have been produced at an earlier 
period. The beds of clay and of sand deposited in 
rivers, lakes, and seas, will necessarily contain the 
remains of those animals which may die and be car- 
* It has been found that emery powder, used for polishing 
glass, takes more than an hour to snik one foot. Now if rnud, 
which is composed of particles twice as coarse, falls at the rate 
of two feet per hour, should be discharged into that part of the 
Gulf Stream which preserves a mean velocity of three miles an 
hour for a distance of 2000 miles, it is evident that in 28 days 
these particles will be carried 2016 miles, and will have fallen 
only to a depth of 224 fathoms. 
