Volcanos and Earthquakes. 49 



often met with close to each other.* The Sieheitgebirge, 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 volcanic scoriae occur on the Roderberg^ 

 opposite to the Siehengebirge, on the left bank of the Rhi7ii\ 

 However different all these rocks are, yet they seem to lead 

 to the conclusion that their ori<^in has been from the very 

 same materials ; for, notwithstanding this difference in their na- 

 ture, it would be easy to form in the S'lebengebirge a gradation 

 from a white trachyte to a compact black basalt.-f* On the other 

 hand, there is every reason to suppose that the nature of the 

 melted matters in the interior is different in different places. 

 If, therefore, after the ejections of melted matters 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 

 unstratificd or volcanic rocks occur, many instances are exhibit- 

 ed, which indicate that these rocks are of very different ages.i" 



If the activity of a volcano ceases, but the channels by which 

 the waters enter remain open, the volcanic action may be re- 

 placed by hot springs. § In this case it is easy to conceive 

 that the meteoric waters, continually sinking into the hot inte- 

 rior, would there assume the surrounding high temperature, and 

 rise again to the surface with a temperature, diminished propor- 

 tionally to the decrease of pressure, eicher through the former 



* The lavas of Vesntiusy of the Solfatanty of Ischia, and of Etna, are quite* 

 different in their nature. 



t See Leonard Horner on the Geology of the Environs of IJonn, in the 

 'I'ransactions of the Geolo<,'icul Society, vol. iv., 2d Scr. p. 43«. Von Buch 

 states tliat in several places in the neighbourhood of Clermont and I'u^ da 

 Dome, a transition from granite into trachyte may be traced, and thvs to 

 have the gradation extended from granite to basalt. 



t L. llorner, 1. c. p. 407. 



§ Von Buch, loco cit. p. 65. A remark of some interest in tracing the 

 counoction of hot springs witli volcanic phenomena is made by Burkart> 

 loco cit. vol. i. p. 31G, viz. that the boiling hot springs in the valley of Pot^ 

 are situated on a line, running from east to west, parallel to the general liii» 

 of volcanos in Mejclco. 



VOL. XXVI. NO. LI. JANUAUV 1839. D 



