502 STONY LAVAS FORMED ON SLOPES. [Cii. XXIX 



beds extend in general over small horizontal spaces, and some of them 

 may possibly be no other than intiusive lavas, in the nature of dikes, 

 more or less parallel to the layers of ejectamenta. Such lavas, when the 

 crater was full, may have forced their way between highly inclined beds 

 of scorise and lapilli. We know that lava often breaks out from the side 

 or base of a cone, instead of rising to the rim of the crater. Neverthe- 

 less one or two of the stony masses alluded to seemed to me to resemble 

 lavas which had flowed out superficially. They may have solidified on 

 a broad ledge formed by the rim of a crater. Such a rim might be of 

 considerable breadth after a partial truncation of the cone. And some 

 lavas may now and then have entirely filled up the atrium, or what in 

 the case of Somma and Vesuvius is called the atrio del cavallo, that is 

 to say, the interspace between the old and new cone. When by the 

 products of new eruptions a uniform slope has been restored, and the two 

 cones have blended into one (see e, d, c, fig. p. 511), the next breaking 

 down of the side of the mountain may display a mass of compact rock of 

 great thickness in the walls of a caldera, resting upon and covered by 

 ejectamenta. Other extensive wedges of solid lava will be formed on the 

 flanks of every volcanic mountain by the interference of lateral, or, as 

 they are often termed, parasitic cones, which check or stop the down- 

 ward flow of lava, and occasionally offer deep craters into w^hich the 

 melted matter is poured. 



By aid of one or all the processes above enumerated w^e may certainly 

 explain a few exceptional cases of intercalated stony beds, in the midst of 

 others of a loose and scoriaceous nature, the whole being highly inclined. 

 But to account for a succession of compact and truly parallel lavas 

 having a steep dip, w^e may suppose that they flowed originally down the 

 flanks of a cone sloping at angles of from 4 to 10 degrees, as in many 

 active volcanoes, and that they acquired subsequently a steeper inclina- 

 tion. It w^ould be rash to assume the entire absence of local disturbances 

 during the growth of a volcanic mountain. Some dikes are seen crossing 

 others of a different composition, marking a distinctness in the periods of 

 their origin. The volume of rock filling such a multitude of fissures as 

 we see indicated by the dikes in Palma must be enormous ; so that, 

 could it be withdrawn, the mass of ejectamenta would collapse and lose 

 both in height and bulk. The injection, therefore, of all this matter in a 

 liquid state must have been attended by the gradual distension of the 

 cone, the increase of which I have elsewhere compared both to the exo- 

 genous and endogenous growth of a tree, as it has been effected alike by 

 external and internal accessions. 



But the acquisition of a steeper dip by such reiterated rendings and 

 injections of a cone is altogethei- opposed to the views of those who 

 defend the upheaval hypothesis, because it draws with it the conclusion 

 that the slopes were always growing steeper and steeper in proportion as 

 the cone waxed older and loftier. Once admit this, and it follows, that the 

 upper layers of solid lava must have conformed to surfaces already inclined 

 at angles of 20, or, in the case of the Caldera of Palma, 28 degrees. 



