253 Natural History of Volcanos and Earthquakes. 



Lancerote, during the eruption in the year 1730, a rent was formed 

 above two German miles in length, on which about twelve con- 

 ical hills rose, whose summits were from 600 to 800 feet in 

 height. 



In like manner basaltic cones, (also porphyritic and granitic 

 hills,) are often seen, which are situated in a line, and of which 

 two or more are connected by rents, which are filled up by ba- 

 salt. Remarkable phenomena of this kind are seen near Mural 

 in Aiivergne.* 



It seems surprising that the same kinds of lava are not always 

 ejected from volcanos. Von Buchf distinguished on Vesuvius 

 alone, eighteen distinct principal kinds of lava ; and old and new 

 lavas of Etna also differ in their characters. The lavas of neigh- 

 boring volcanos are often very different from each other. In like 

 manner, unstratified rocks of very different natures are often met 

 with close to each other.J The Siebengebirge, near Bonn, offer 

 remarkable instances of this kind. There, trachytes, trachyte 

 tuffs, basalts, and basalt tuffs, are met with close to each other. 

 Basalt dykes traverse the trachyte and the trachyte tuffs, and vol- 

 canic scorisB occur on the Roderberg, opposite to the Siebenge- 

 birge, on the left bank of the Rhine. However different all these 

 rocks are, yet they seem to lead to the conclusion that their ori- 

 gin has been from the very same materials ; for, notwithstanding 

 this difference in their nature, it would be easy to form in the 

 Siebengebirge a gradation from a white trachyte to a compact 

 black basalt. <§> On the other hand, there is every reason to sup- 

 pose that the nature of melted matters in the interior is different 

 in different places. If, therefore, after the ejection of melted mat- 

 ters existing in a particular spot, new eruptions will take place 

 only when such matters flow from remote places towards this 

 spot, we can hence easily conceive how different lavas may be 

 ejected at different times. In the Siebengebirge, as well as in 

 other places where unstratified or volcanic rocks occur, many in- 



* Leonhard's Basalt Gebilde, t. i, p. 408. t Beobachtungen. &c. t. ii, p. 174. 



X The lavas of Vesuvius, of the Solfatara, of Ischia, and of Etna, are quite differ- 

 ent in their nature. 



§ See Leonard Horner, on the Geology of the Environs of Bonn, in the Trans- 

 actions of the Geological Society, vol. iv, 2d Ser. p. 438. Von Buch states that in 

 several places in the neighborhood of Clermont and Puy de Dome, a ti-ansition 

 from granite into trachyte may be traced, and thus to have the gradation extended 

 from granite to basalt. 



