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THE WONDERS OF GEOLOGY. Lect. VIII. 



miles in diameter, and from two to three thousand feet in 

 the height of its bounding precipices, which in most places 

 are nearly perpendicular. This remarkable area appears to 

 have resulted from the giving way and subsidence of part 

 of the crust of the volcano, from some violent action in the 

 interior, which occasioned the sudden removal of an enor- 

 mous mass of mineral matter.* This plain is encircled by 

 subordinate volcanic mountains, some of which are covered 

 by forests, while others are bare and arid like many of the 

 cones of Auvergne. The walls or cliffs surrounding this 

 depression, are formed of successive layers of lava of variable 

 thickness, with interposed beds of tuff, ashes, and igneous 

 conglomerates of different colours and degrees of fineness. 

 They slope downwards towards the sea at an angle of 

 from twenty to thirty degrees, and have evidently been 

 formed at various intervals by successive eruptions from the 

 top of the mountain, and were continuous before the 

 subsidence took place which gave this region its present 

 character. 



The perpendicular sides of this natural amphitheatre are 

 everywhere marked by vertical walls or dikes, which 

 not only intersect the concentric sheets of lava and 

 tuff, but standing out in bold relief, like prodigious but- 

 tresses, impart a most extraordinary character to the 

 scene ; the greater induration of these intruded dikes 

 having enabled them to resist the denuding action which 



* Sudden depressions of the surface of the land, are not unfrequent 

 concomitants of subterranean movements, and occasionally produce the 

 most frightful catastrophes. In 1772, Papandayang, one of the largest 

 volcanoes in the Island of Java, suddenly sunk down, with a terrific 

 noise, and an area fifteen miles long and six wide was swallowed up ; 

 many hundred persons and forty villages were destroyed : being 

 either engulfed with the sunken mass, or overwhelmed by the volcanic 

 matter that issued forth, and spread over the surrounding country to a 

 considerable distance. The original mountain was diminished 4,000 

 feet in height. 



