138 IGXEOUS AGENCIES. 



another test of crust-movements, viz., the phenomena of river-beds. In 

 a rising area the rivers cut rapidly deeper ; in a subsiding area they fill 

 up their old beds and rise to higher level. In this way we know that 

 the Plateau and the Sierra regions have greatly risen in comparatively 

 recent times, and are still rising, while the New England region has 

 recently subsided, though probably is not still subsiding. The evidence 

 of this will be given hereafter. 



Theories of Elevation and Depression. 



It is evident that observation only determines changes of relative 

 position of sea and land. These changes may be the result of rise and 

 fall of sea, or rise and fall of land. The popular mind naturally at- 

 tributes them to the rise and fall of the sea, as the more unstable ele- 

 ment. But, by the principle of hydrostatic level, it is clearly impos- 

 sible that the ocean should rise or fall permanently at one place without 

 being similarly affected everywhere. It is certain, therefore, that the 

 changes we have described above, being in different directions in dif- 

 ferent places, must be due to movements of the solid crust. Neverthe- 

 less, it is also true that any increase in the height and extent of the 

 whole amount of land on the globe must be attended with a correspond- 

 ing depression of the sea-bottoms, and therefore an actual subsidence 

 of the sea-level everywhere. Hence, if it be true, as is generally be- 

 lieved, that the continents have been, on the whole, increasing in ex- 

 tent and in height, in the course of geological history, then it is true 

 also that the seas have been subsiding, and that therefore the relative 

 changes are the sum of these two. 



Admitting, however, that the actual increase of land at the present 

 time is imperceptible, or at least very small in comparison with the 

 oscillatory movements described above, we may look upon the sea-level 

 as fixed ; this statement being sufficiently correct when regarding the 

 subject from the physical point of view, though untenable when re- 

 garded from the geological point of view. Admitting, then, the fixed- 

 ness of the sea-level, what are the causes of the gradual movements of 

 the solid crust ? 



Babbage's Theory. — Babbage believed that, in the vicinity of volca- 

 noes, the rise and fall of ground were due to the expansion and con- 

 traction of rocks by heating and cooling. The re-elevation of the tem- 

 ple of Serapis occurred apparently soon after the eruption which formed 

 Monte Nuovo (Fig. 111). It is not improbable that this re-elevation 

 was the result of the heating and vertical expansion of the rocks to 

 great depth, caused by the eruption of the interior heat at this point. 

 A very small elevation of temperature of rocks several miles thick 

 would be sufficient to produce a vertical expansion of twenty feet. 



Other cases, such as the rise of sea-margins at a distance from 



