72 



EARTHQUAKES, FISSURES, ETC. 



crater ejecting volcanic matter and vapour, the surrounding sea being rendered turbid 

 with floating cinders and scoria, and encumbered with dead fish. 



5thly. The gradual increase of the island to 200 feet in height, and eventually to a 

 circumference of three miles. 



6thly. Its rapid diminution to a circumference of about 700 yards, when last exa- 

 mined by the French naturalists 1 , under the erosive action of the waves and currents, 

 which its loose fragmentary materials could not withstand. 



7thly. Its final disappearance in less than three months from the period of its emer- 

 gence. 



8thly. The spot being surveyed in 1833, a year after its disappearance, the whole 

 submarine remains of this mass, which had been raised from 600 feet beneath to 200 

 feet above the sea level, was reduced to a dangerous reef about eleven feet under water, 

 in the centre of which was a black volcanic rock (probably a remnant of the solid lava 

 of eruption), surrounded by banks of black stones, scoriae, and sand. 



In these well-recorded facts, therefore, we perceive that the eruption and demolition 

 of this island, the apex of a cone 800 feet in height, must have covered the bottom of 

 the sea to a great extent with detritus of volcanic ashes, scoriae, and lava, destroying 

 countless marine animals, and mixing up their remains with the previous materials 

 of the bed of the sea to form subaqueous deposits. We shall hereafter advert to the 

 value of such a modern analogy in explaining many geological phenomena, particularly 

 in the bedded trap rocks of the Silurian System. 



This slight sketch of volcanic eruptions may suffice to explain, that they could not 

 have occurred without producing great and striking changes in physical geography. 

 Important, however, as the volcano must be considered as a great natural means of de- 

 struction and renovation, the earthquake, its constant accompaniment, operates perhaps 

 still more directly as an agent of change, by elevating some tracts and by depressing 

 others, causing great rents and fissures in the strata, and giving rise to powerful varia- 

 tions of tides and currents, by wdiich solid materials are transported to distant places. 

 The volcano and the earthquake are in truth dependent on the same cause, and are but 

 the outward signs of internal heat. The one is the " safety valve," by which heated 

 matter escapes at intervals from the interior, the other is the shock which lacerates the 

 solid ribs of the earth when that heated matter and its vapours, are denied an access to 

 the atmosphere. One important task, therefore, of the geologist is to read off the 

 proofs of these eruptions and earthquakes amid the ancient monuments which sur- 

 round him on the surface of the earth ; and by examining them he learns, that from the 

 remotest time there have been volcanic eruptions, and that the framework of the planet 

 has been repeatedly subjected to intermittent violence and fracture. He perceives that 



1 See a most instructive account of these phenomena by M. Constant Prevost, Notes surl'Ile Julia, Mem. de 

 la Soc. Geologique de France, vol. ii. p. 91. 



