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 lations 



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ch. xxxiil] peeseevation of balance of dey land. 



237 



iucujJibent mass, we may easily conceive liow the country 

 may remain permanently nplieaved. For in some places the 

 fractm^ed rocks below may have assumed an arched form, or 

 lava may have been driven into fissures, in which it may 

 consolidate, and afford an enduring support to the foun- 

 dations of the newly raised strata. 



The sudden subsidence of limited areas of land may be 

 occasioned by subterranean caverns giving way, when gases 

 are condensed, or when they escape through newly-formed 

 crevices. The subtraction, moreover, of matter from certain 

 parts of the interior, by the flowing of lava, and of mineral 

 springs, must, in the course of ages, cause vacuities below, 

 so that the undermined surface may at length fall in or be 

 slowly depressed. In this 



the geographical connection which seems to exist between 

 areas of elevation and of subsidence, a deep sea being often 

 contiguous to elevated land. 



Balance of dry land, how preserved. — It will appear, from the 

 historical details above given, that the force of subterranean 

 movement, whether intermittent or continuous, whether with 

 or without disturbance, does not operate at random, but is 

 developed in certain regions only ; and although the altera- 

 tions produced during the time required for the occurrence 

 of a few volcanic eruptions may be inconsiderable, we can 

 hardly doubt that, during the ages necessary for the for- 

 mation of large volcanic cones, composed of thousands of 

 lava currents, shoals might be converted into lofty moimtains, 

 and low lands into deep seas. 



manner 



former chapter (Vol. I. p. 327), I have stated that 



force 



may 



s 



1 the aqueous labouring incessantly to reduce the in- 

 equalities of the earth's surface to a level, while the igneous 

 are equally active in renewing the unevenness of the surface. 

 By some geologists it has been thought that the levelling 

 power of running water was opposed rather to the elevating 

 force of earthquakes than to their action generally. This 

 opniion is, however, untenable ; for the sinking down of the 

 bed of the ocean is one of the means by which the gradual 

 submersion of land is prevented. The depth of the sea cannot 



Mt 



