134 



UPHEAVAL AND SUBSIDENCE 



[Ch. VII. 



C* 



Nor 



do not acquire in a few days or hours an additional hei 



ten, 

 ght 



om 



me- 



movm 



because the antagonist power, or the strength, toughness, and 



sufficient 



as to allow the volcanic energy an indefinite time to accumu- 

 late. Instead of the explosive charge augmenting in quantity 

 for countless ages, it finds relief continuously, or by a suc- 

 cession of shocks of moderate violence, so as never to burst 

 or blow up the covering of incumbent rock in one grand 



paroxysmal convulsion. 



most 



displays an intermittent and mitigated intensity, beii 

 permitted to lay a whole continent in ruins. Hence 



mer 



from the same 

 1 similar earthen 



of years along certain areas or zones of country. Hence 



numerous monuments 



m 



sures formed in distinct ages, and often widened and filled at 

 different eras. 



Among the causes of lateral pressure, the expansion by 

 heat of large masses of solid stone intervening between others 

 which have a different degree of expansibility, or which hap- 

 pen not to have their temperature raised at the same time, 

 may play an important part. It may also happen that hot 

 vapours 

 matters in solution, may permeate rocks, and while they rise 



or thermal waters charged with various mineral 



to new chemical combinations and a metamorphic structure, 

 may augment their volume. We know also that at every 

 period some countries have sunk down hundreds or thou- 

 sands of feet below their original level, and we can hardly 

 doubt that much of the bending of pliant strata, and the 

 packing of the same into smaller spaces, has been occasioned 

 by such subsidence. Whether the failure of support be pro- 

 duced by the melting of porous rocks, which, when fluid, and 

 subjected to great pressure, may occupy less room than before 



? 



? 



or which, by passing from a pasty to a crystalline condition 

 may, as in the case of granite, according to the experiments 







o: 



Qi 



gp 



y 



of 



*h 



iiio 





