252 Natural History of Voleanos 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 wp by ba- 
salt. Remarkable phenomena of this kind are seen near Murol 
in Auvergne.* 
It seems surprising that the same kinds of lava are not always 
ejected from volcanos. Von Buch+ distinguished on Vesuvius 
alone, eighteen distinct principal kinds of lava; and old and new 
lavas of Hina 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.{ 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. 
lt dykes traverse the trachyte and the trachyte tufts, and vol 
canic scorie 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 theit 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 Stebengebirge, as well as in 
other places where unstratified or voleanie rocks occur, many in- 
et, ae 
* Leonhard’s Basalt Gebilde, t. i, p. 408. t Beobachtungen, &c. t. ii, p. 174. 
t 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 Lape ” 
several places in the neighborhood of Clermont and Puy de Dome, 4 pee 
ten 
from granite into trachyte may be traced, and thus to have the gradation ex 
from granite to basalt. 
