166 



FORMATION OF DELTAS. 



ved from the disintegration of the solid 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 sink one foot. Now if mud, 

 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. 



