502 



ACTION OF THE SEA ON THE BRITISH COAST. [Ch. XX. 





• 



energy shift perpetually. The height to which the tides rise 

 and the violence and velocity of currents, depend in a great 

 measure on the actual configuration of the land, the contour 

 of a long line of continental or insular coast, the depth and 

 breadth of channels, the peculiar form of the bottom of seas, 

 in a word, on a combination of circumstances which are made 



» 



to vary continually by many igneous and aqueous causes, 

 and, amongst the rest, by the tides and currents themselves. 

 Although these agents, therefore, of decay and reproduction 

 are local in reference to periods of short duration, such as 

 those which history embraces, they are nevertheless universal, 

 if we extend our views to a sufficient lapse of ages. 



currents. — After these 



Destroying and transporting power of 



preliminary remarks on the nature and causes of currents, 

 their velocity and direction, we 



may 



consider their 



* 



action on the solid materials of the earth. We shall find 

 that their efforts are, in many respects, strictly analogous to 

 those of rivers. I have already treated in the third chapter, 

 of the manner in which currents sometimes combine with 

 ice, in carrying mud, pebbles, and large fragments of rock to 

 great distances. Their operations are more concealed from our 

 view than those of rivers, but extend to wider areas, and are 

 therefore of more geological importance. 









ACTION OP THE SEA ON THE BRITISH COAST. 



Shetland Islands. — If we follow the eastern and southern 

 shores of the British Islands, from our Ultima Thule in Shet- 

 land to the Land's End in Cornwall, we shall find evidence 

 of a series of changes since the historical era, very illus- 

 trative of the kind and degree of force exerted by tides and 

 currents co-operating with the waves of the sea. In this 

 survey we shall have an opportunity of tracing their joint 

 power on islands, promontories, bays, and estuaries ; on bold, 

 lofty cliffs, as well as on low shores ; and on every description 

 of rock and soil, from granite to blown sand. 



The northernmost group of the British Islands, the Shet- 

 land, are composed of a great variety of rocks, including 





mica 



