PRINCIPLES OF GEOLOGY. 25 



Whoever has observed the sea-shore, with attention, is aware that the 

 sand and pebbles, which constitute the beach, undergo continual change 

 of place. The little heaps of gravel which are sometimes ranged in lines 

 according to the height of the tide, are at other times strewed over the 

 sand. According as the tide sets along the shore, the pebbles are driven 

 onward progressively, accumulated in little quiet recesses of the cliffs, 

 and heaped together in profusion in the larger bays. The large angu- 

 lar stones usually remain near the spot where they fell, but the smaller 

 ones, after being rolled about by the waves till they become pebbles, are 

 subject to the same progressive motion as the ordinary gravel ; the 

 sand travels in the same direction, and the finer particles of clay, mixed 

 with and suspended in the water, are transported far aAvay, and finally 

 deposited on the marshes ; and thus by the fall of the heights ma- 

 terials are provided for the extension of the lower ground. The 

 wasted cliffs of Holderness have furnished the pebbles which compose 

 the long projecting point of Spurn, and part of the silt which enriches 

 the marshland along the rivers Ouse, Aire, Dun, and Trent. The sea 

 engulphs but little of what falls from the ruin of its boundaries ; its 

 effect is to abate the high, and to raise and extend the low parts of its 

 shores. When the latter part of this process has proceeded so far that 

 the marshes are dry at intervals, man exerts his enterprising industry, 

 and defends the new land by a bank. If this be made too abrupt, the 

 ocean indignantly washes it away, and reclaims his ancient domain ; but 

 a long gradual slope of pebbles and sand averts the fury of the sea, and 

 protects, though with a moving barrier, the lands within, above which, 

 in storms, the waves hang suspended and threatening destruction, but 

 dash their spray and fling their foam in vain. 



ACTION OF iiiVERS, &c. Imperceptible as is the reduction of 

 mountains and hills by rains and rivulets, yet the matter thus collected, 

 by constant attrition, assumes an important character, when concentrated 

 along the margins of rivers, and changes the appearance of the vallies. 

 In proportion to the magnitude of the stream, the altitude of its sources, 

 and the nature of the country through which it flows, the effects are 

 more or less considerable. But they every where tend to the same 



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