48 Prof. Bischof on Natural History of 



often very considerable. A rent of this description produced by 

 one of the most violent eruptions of Etna, viz. that of the 11th 

 May 1669, was 2^ German miles in length, and occupied al- 

 most one-third of its height. Scrope* saw distinct traces of it 

 near Nicolosi so late as the year 1819. Even the rent formed 

 during the eruption in 1794 on the declivity of Vesuvius, to- 

 wards Torre del Greco, was, according to Von Buch, 3000 

 feet in length, and according to Breislak 237 feet in breadth 

 at its upper edge. 



Other volcanos afford instances of the formation of rents 

 and hills. Thus during the most violent eruption of Scaptar 

 Jdkul on Iceland in 1783, a rent e^'ght Enghsh miles in length 

 was formed in a plain at the foot of the mountain. Three 

 craters, from which immense quantities of lava flowed out, rose 

 in the direction of the rent, and afterwards a fourth appeared 

 below the sea in the same direction, and at a distance of thirty 

 miles, the eruption of which formed an island, which afterwards 

 disappeared again t. Similar phenomena took place in the 

 same year in the island of Java. And Von Buchf informs 

 us, that in the island of Lancerote, during the eruption in the 

 year 1730, a rent was formed above two German miles in length, 

 on which about twelve conical 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 

 basalt. Remarkable phenomena of this kind are seen near 

 Murol in Ativergne. |i 



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

 ejected from volcanos. V^on Buch § 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- 

 bouring volcanos are often very different from each other. In 

 like manner, unstratified rocks of very different natures are 



* Considerations, p. 158. t Ibid, p. 154. 



:{: Leouhard's Taschenbucli, 1824, t. ii. p. 439. 



II Leonliard's Basalt Gebilde, t. i. p. 408. 



§ Bcobachtungen, &c. t. ii. p. 174. 



