130 Volcano,?. 



inferior focus, is greatest at this time, and such a phenomenon 

 will, therefore, probably soon occur. By this, the mountain is 

 once more gutted. The crater left by these paroxysms, is usu- 

 ally a deep elliptical chasm, resulting from the enlargement of 

 the original fissure of eruption, by the violent ascent of the elas- 

 tic fluids. Examples are given, in great numbers, of such cra- 

 ters, and of the changes they have undergone. Paroxysmal 

 eruptions of this kind have, in some instances, blown into the 

 air the whole frame of the mountain, and replaced it by a lake. 

 The eruption of Vesuvius in 79 A. D., is supposed to have thus 

 shattered one-half of the original cone of Somma, burying Pom- 

 peia and Herculaneum under its fragments. Such craters are 

 often occupied afterwards by lakes, particularly where the con- 

 glomerates are of a feldspathose nature, since these form, by 

 mixture with water, a mud or clay, which is impervious to wa- 

 ter. Other lake-basins, in volcanic districts, are formed by ex- 

 plosions from a deep focus, on some fresh point of the earth's 

 surface, as are some in the Eiffel and Auvergne, which have 

 been drilled in this manner through rocks of greywacke slate 

 and granite, the fragments of which are scattered on all sides. 

 The bursting of lakes in the interior of volcanic craters, gives 

 rise to what the author calls, Eluvial deposits ; the trass of the 

 Rhine, the moya of America, and some tufas, are attributed to 

 this origin. These conglomerates sometimes assume a division- 

 ary structure on desiccation, and set very firmly, so as to be used 

 as building-stone ; the tufas, which have not been thus forcibly 

 mixed with water, seldom cohere so compactly. Organic re- 

 mains occur, of course, also in these conglomerates, particularly 

 wood. The primary vent of a volcano is sometimes shifted later- 

 ally ; the original crater remaining extinct, and usually reduced 

 to the state of a solfatara. Teneriffe and Bourbon are noted ex- 

 amples of this circumstance." 



One of the most remarkable examples of the explosion of 

 an entire volcanic mountain happened in 1688 in the island 

 of Timor one of the Molluccas. 



The whole mountain which was before this continually 

 active, and so high that its light was visible, it is said, three 

 hundred miles off, was blown up and replaced by a concav- 

 ity now containing a lake. 



" The next chapter is upon Subaqueous Volcanos, which are 

 supposed by Mr. Scrope to be much more numerous than is gene- 

 rally imagined. Indeed, all insular volcanos, (and most of those 

 we are acquainted with are of this character,) have been origi- 

 nally produced by submarine vents. 



