Mr Poulett Scrape's Considerations on Volcanos. 349 



currents to a distance. This was the origin of the stratified tufas of Cam- 

 pania, and the environs of Rome. In Hungary, pumice conglomerates al- 

 ternate with tertiary limestone, as basaltic peperinos do in Auvergne, the 

 Vicentine, and the Val Demona in Sicily. 



The craters of volcanic mountains are subject to a series of changes, by 

 which they are alternately filled up and emptied again. The large crater, 

 left by any paroxysmal eruption, is gradually filled by the accumulating 

 products of minor eruptions, until it is replaced by a convexity of summit. 

 This form is, as we have seen, the most favourable to the permanence of 

 the eruptive process ; but, at the same time, the quantity of matter accu- 

 mulated above the focus, and, therefore, the obstruction to the escape of 

 the caloric in as quick a ratio as it is received there from below, is at its 

 maximum ; consequently, the probability of the recurrence of a paroxysmal 

 eruption, from this inferior focus, is greatest at this time, and such a phe- 

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

 once more gutted. The crater left by these paroxysms, is usually a deep 

 elliptical chasm, resulting from the- enlargement of the original fissure of 

 eruption, by the violent ascent of the elastic fluids. Examples are given, 

 in great numbers, of such craters, and of the changes they have under- 

 gone. 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 Pompeia and Hercula- 

 neum under its fragments. Such craters are often occupied afterwards by 

 lakes, particularly where the conglomerates are of a feldspathose nature, 

 since these form, by mixture with water, a mud or clay, which is imper- 

 vious to water. 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, arc attributed to 

 this origin. These conglomerates sometimes assume a divisionary struc- 

 ture on desiccation, and set very firmly, ?o as to be used as building-stone ; 

 the tufas, which have not been thus forcibly mixed with water, seldom 

 cohere so compactly. Organic remains occur, of course, also in these con- 

 glomerates, particularly wood. The primary vent of a volcano is some- 

 times shifted laterally ; the original crater remaining extinct, and usually 

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

 amples of this circumstance. 



The next chapter is upon Subaqueous Volcanos, which are supposed by 

 Mr Scrope to be much more numerous than is generally imagined. Indeed, 

 all insular volcanos (and most of those we are acquainted with are of this 

 character,) have been originally produced by submarine vents. 



The observed instances of eruptions from the sea arc indeed few in num- 

 ber. Our author mentions those off St Michael, one of the Azores in 1 <i:{s. 



