THEORY OF THE EARTH. 49 



the beach. By Jhis means, downs, or ranges of low 

 sand-hills, are formed along the coast. These, if 

 not fixed by the growth of suitable plants, either 

 disseminated by nature, or propagated by human in- 

 dustry, would be gradually, but certainly, carried 

 towards the interior, covering up the fertile plains 

 with their sterile particles, and rendering them 

 unfit for the habitation of mankind; because the 

 same winds which carried the loose dry sand from 

 the shore to form the downs, would necessarily 

 continue to drift that which is at the summit farther 

 towards the land. 



12. Of the Formation of Cliffs, or steep Shores. 



On the other hand, when the original coast hap- 

 pens to be high, so that the sea is unable to cast 

 up any thing upon it, a gradual, but destructive 

 operation is carried on in a different way. The in- 

 cessant agitation of the waves wears it away at the 

 bottom, and at length succeeds in undermining it, 

 causing the upper materials to slide and tumble 

 down, and converting the whole elevation into 

 steep sloping bluffs or cliffs. In the progress of this 

 change, the more elevated materials which tumble 

 down into the sea, have their softer parts washed 

 out and carried away by the waves; while the 

 harder parts, continually rolled about in the agi- 

 tated water, form vast collections of rounded stones 

 and pebbles* and of sand of various degrees of fine- 

 ness, which at length accumulate into sloping banks 



7 



