62 Prof. Bischof on the Natural History of 



rents of more or less considerable width ; and partly form 

 dykes, or mountains of some magnitude ; or raise up or break 

 through the upper strata of the earth. Thus Von Buch* in- 

 forms us that on the island of Lancerote^ during an eruption 

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

 on which about twelve conical hills had risen, whose summits were 

 from 600 to 800 feet in height. In like manner basaltic cones 

 (also even porphvritic and granitic hills) 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 Auvergiie. 



We have abundance of proofs of the rising of masses of melt- 

 ed or at least semifluid matter, J out of the interior of the earth, 

 in the filling up of dykes with compact crystalline rocks, in all 

 of which, as in the rocks of undoubted volcanic origin, felspar 

 forms a necessary and principal ingredient.§ We find these 

 rocks in contact with all the stratified and superficial forma- 

 tions, even with those which are going on at the present day. 

 But similar masses, which have evidently flowed in streams 

 from craters, are also found in positions which shew that they 

 must have risen from the interior of the earth, after the forma- 

 tion of the stratified rocks, and found their way into fissures, 

 which in many cases do not reach the surface. Thus, granite, 

 syenite, trachyte, the porphyries, the greenstone, and so on, up 

 to the basalts, form dykes in the stratified rocks as well as in one 

 another. They also not unfrequently appear in beds between 



* Leonard'^ Tasclienbuch, 1824, Abtli. ii, p. 439. 



t Leonhard die Basalt Gebilde, t. i. p. 408. 



Z Cones of basalt, trachyte, and pliondlite, whose inclination is often very 

 considerable, cannot have risen in such a thin liquid state, as that in whicli 

 lava issues from volcanos ; for, according to the observations of Elie de 

 Beaumont already mentioned, lava streams having an inclination of only G° 

 cannot form a continuous mass. See on this subject Leonhard, loco cit. t. i. 

 p. 417, &c. 



§ Felspar may certainly be considered as a characteristic sign of an igne- 

 ous origin in rocks, as this mineral is never found in rocks, in the forma- 

 tion of which the action of volcanic power can be proved to have been 

 wholly excluded. 



