\ 



238 



PRESEEVATION OF BALANCE OF DEY LAND. [Ch. XXXIII. 



be increased at any one point without a nniversal fall of tlie 

 waters, nor can any jjartial deposition of sediment occur 



displacement 



imperce 



degree, even to the antipodes. The preservation, therefore 



of the dry land may sometimes be effected by the subsidence 



of part of the earth's crust (that part, namely, which is 



an upheavino- 



manner 



covered by the ocean), and in like 



movement must often tend to destroy land ; for if it render 



mor 



submero-e 



If the dimensions of the planet have remained uniform 



contemplate in geology, it would 

 be necessary to suppose that the amount of depression caused 

 by subterranean heat must exceed that of elevation, otherwise 

 there would not be a perpetual restoration of those inequa- 

 lities of the earth's surface which the levelling power of 

 water tends to efface. It would be otherwise if the action of 

 volcanos and mineral springs were suspended 3 for then 

 the forcing outwards of the earth's envelope ought to be no 

 more than equal to its sinking in. 



To understand this proposition more clearly, it must be 

 borne in mind, that the deposits of rivers and currents 

 probably add as much to the height of lands which are 

 rising, as they take from those which have risen. Suppose 

 a large river to bring down sediment to a part of the ocean 

 2,000 feet deep, and that the depth of this part is gradually 

 reduced by the accumulation of sediment till only a shoal 

 remains, covered by water at high tides ; if now an upheaving 

 force should uplift this shoal to the height of 2,000 feet, the 



1. But had the 



mountain 



movement 



& 



same 



before the sediment 



of tlie bottom of tlie sea 



filled it lip; then, 



instead of changing a shoal into a mountain 2,000 feet high, 

 it would only have converted a deep sea into a shoal. 



It appears, then, that the operation of the volcanic or 

 subterranean forces is often such as to cause the levelling 

 power of water to counteract itself; and, although the idea 

 may appear paradoxical, we may be sure, wherever we find 



i 



> 





r 



n 



) 



CH' 





11 :i 



suc 



hill' 



the 



tion 

 rent 



coiitin*' 



from 



li 



»• 



As 



ted by 



tratified 



crea 



s 



1 



re 



of earbonat 

 which sprii 

 and portion 

 external ace 

 dimensious 



of depre««io 

 the elevati* 

 of the eartj 

 the surface 



subsidence 

 of depre-I^ 

 ciples, siiie^ 

 either to pre 

 diminution ^ 

 ^^e subtrac 



f 



Mineral 



8pr 



^^s^es by „ 

 '^i^i-at.,1 CO 



If 



queues 



(?etl 



tr ''^ Wv. 



tie. 



liei,> 



'^ight 



w 





H 



of 



of 



the 



