88 THE B0TT03I OF THE SEA. 



rising of which must seriously modify the aspect of 

 our globe. Stormed without ceasing by the waves, 

 the most iron-bound coast must gradually give way, 

 but with results which vary according to the nature 

 and formation of the coast-line. 



The heavier debris is naturally disposed to con- 

 tinue its descent till it reaches the greatest depths. 

 The action of gravitation is, in this respect, favoured 

 by the action of the currents. We have explained 

 in a previous chapter that the crust of the globe is, 

 so to speak, ploughed up in furrows, the sides of 

 which combine to form basins or cavities which are 

 separated from each other by ridges of rock more or 

 less sharply set. In geographical science these fur- 

 rows are, one and all, regarded as the basins of rivers. 

 The waters flowing in them naturally tend to the 

 lower parts, and in those lower parts they combine 

 to form perhaps a single current. This current flows 

 down to a still lower level — that of the sea. Nor 

 even here does the conformation lose its character 

 of a furrow or basin. We follow it under the sea 

 until we have descended so deep that we seem to 

 have reached a central point from which it is impos- 

 sible to advance in any direction without reascend- 

 ing. Using this lowest point as a centre, we draw a 

 line around it at a greater or less distance, and that 

 line indicates the circumference of what we have 



