570 



EFFICACY OF EIVEES AND CUEEENTS AS 



[Ch. XXII. 



banks during heavy gales, causing the sands to shift their 

 position, in which case the strata, when re-deposited, must 

 greatly resemble the so-called crag of Norfolk and Suffolk. 

 Nor can there fail, in the course of ages, to be spots where 

 some of the unconsolidated older tertiary formations, such 

 for example, as the Bagshot sand and London clay, will be 

 denuded with as much facility as the modern sand-banks ; 

 such denudation would inevitably take place during oscilla- 

 tions in the level of the bottom of the sea, like those which 

 we know to have occurred during and since the Glacial 

 Epoch. Whenever the sea scooped out such channels in the 

 ancient strata, fossil shells of extinct species would be 

 mingled with recent ones, and both the one and the other 

 would often be more or less rolled. As the bones also of the 

 elephant and other extinct mammalia are occasionally 

 dredged up with oysters attached to them, from the bed of 

 the sea between Suffolk and the Netherlands, such fossil 

 bones would be occasionally included in the new formations. 

 The chief difference in character between the Pliocene and 

 modern strata will consist in the intermixture in the latter 

 of works of art together with the bones of man; monuments 

 of hundreds, nay thousands, of wrecked vessels, which in the 

 last twenty centuries have sunk on these banks, and so im- 

 peded for a time the free passage over them of shelly sand, 

 as to produce shoals reaching within thirty feet of the 

 surface. 



So great is the quantity of mud held in suspension by the 

 tidal current on our shores, that it is found useful artificially 

 to introduce the water into certain lands reclaimed from the 

 sea and which are below the level of high tide ; and by re- 

 peating this operation, which is called ' warping/ for two or 

 three years, considerable tracts have been raised, in the 

 estuary of the Humber, to the he 

 a current, charged with such materials, meets 



If 



pressions in the bed of the ocean, it must often fill them up ; 



meets 



sediment 



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and denuding agents. — I have said (p. 565) that the action of 







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