Areas of Elevation and Depression. 135 



Both may be said to move round their common centre of gravity, 

 which is that of the whole planet, and, in consequence, we may 

 conceive a gravitation of the whole mass of the ocean in one direc- 

 tion — for example, in favour of the southern hemisphere, which 

 would result in the sinking of the level of the ocean — i.e., the crea- 

 tion of dry land in the northern hemisphere. The great plains of 

 the present continents, such as the plain which stretches from 

 the English Channel to the coast of Siberia, and the great 

 North American plain, have all the appearance of having 

 been converted into dry land, not through the action of a 

 subterraneous agency which lifted them above the level of the 

 sea, of which action they bear little or no trace, but in con- 

 sequence of the retreat of the ocean ; and the great rivers which 

 now wind their course through these plains have carved out 

 their bed, not through strata previously deposited by themselves 

 in a former geological epoch, but through strata deposited at the 

 bottom of former oceanic basins and great inland seas. 



Or, supposing the quantity of oceanic waters to be constant, 

 the bed of the ocean may be either deepened or rendered more 

 shallow, or its total area made wider or narrower, in consequence 

 of submarine denudation, the formation of new seas, the accumu- 

 lation of fresh deposits, and the uplifting or depression of wide 

 areas by subterraneous forces. Any such alteration in the con- 

 tour of its bed, of the occurrence of which in the past and in the 

 present there is ample evidence, may either raise or lower the 

 level of its surface, and it has already been shown how pro- 

 foundly the distribution of land and water would be affected by 

 a change of level amounting to the merest fraction of the total 

 depth of the ocean. 



Formation and Transformation of Continents. — If we 

 suppose that at one time the ocean covered the whole surface of 

 the earth, the plateaux accumulated in consequence of the 

 unequal distribution of soli I matter by the thermal oceanic cur- 



