Volcano^ and Earthquakes. G5 



recent eruptions at Ponohohoa in Owhyliee^^ establish the pos- 

 sibility of eruptions through rents. 



If alterations in the adjacent rocks, or other phenomena aU 

 ready mentioned, are not observed, we may infer that the ele- 

 vations havQ taken place in a solid state. Notwithstanding this 

 solidity, the highly elastic and exceedingly hot vapours may 

 certainly cause considerable chemical alterations in the elevated 

 masses, as well as in the adjacent rocks. 



It is impossible to determine any regular order of succession 

 in the elevation of the pyrogenetic rocks. They occur in every 

 period of the stratified formations. Older ones have very com- 

 monly received those of more recent date into their fissures. 

 There scarcely exists a single unstratified rock which is not 

 somewhere to be found filling up dykes in granite. Basalt- 

 tlykes traverse many unstratified rocks, such as trachyte, con- 

 glomerate, and others. In Iceland^ tufa is found alternating 

 with slaggy lava ; and dykes of a porous trachytic rock traverse 

 the tufa of Stromholi and Vidcanello in the Lipari Islands^ 

 &c.t 



Masses of melted r^atter will break through the bottom of 

 the sea more easily, 1 -cause resistance is there the least consi- 

 derable. To this may be ascribed the frequent elevation of 

 islands from the bottom of the sea, not only in historical times, 

 but also at the present day, and under the eyes of observers, in 

 whom the utmost confidence may be placed. The most extra- 

 ordinary and instructive island in this respect is Santorin, be- 

 cause it unites the whole history of volcanic islands and islands 

 of elevation. A more beautiful, regular, and perfect crater of 

 elevation is not to be found, than in the space which is almost 

 entirely surrounded, by the inner circle of Santorin (wliich en- 

 compasses more than one-half of it), and by its continuation 

 as exhibited in the islands of Therasia and Aspronisi.i Here 

 it is probable that the clay-slate was broken through and up- 



* Pog<rendorff's Auual. t. ix. p. 141. 



t De la Beclie, Ilandbucli der Geoguosie Von v. Decben. Berlin, 1832. 

 Absclmitt xi. 



t Von Biicb iu Poggondorff's Annal. v. x. p. 172. See the drawing iu 

 his splendid atlas, and the sketch in these annal. v, xxiv. p. 1. 



VOL. XXVI. NO. LI. JANUAKY 1839- ^ 



