636 STONY LAVAS FORMED ON SLOPES. [Ch. XXIX. 



to the rim of the crater. Nevertheless 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 erup- 

 tions a uniform slope has been restored, and the two cones have blend- 

 ed into one (see e, d, c, fig. p. 645), 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 downward flow 

 of lava, and occasionally offer deep craters into which the melted mat- 

 ter is poured. 



By aid of one or all the processes above enumerated we may cer- 

 tainly 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, we may suppose that they flowed 

 originally down the flanks of a cone sloping at angles of from 10 to 

 15 degrees, or even much more, as in many active volcanoes. They 

 may also have acquired subsequently a still steeper inclination, for it 

 would 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 multi- 

 tude 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 enormous; so that, could it be withdrawn, the mass of ejectamenta 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 exogenous and endogenous growth 

 of a tree, as it has been affected alike by external and internal acces- 

 sions. 



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

 injections of a cone is altogether opposed to the views of those who 

 defend the upheaval hypothesis, because it draws with it the conclu- 

 sion that the slopes were always growing steeper and steeper in pro- 

 portion 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. 



For this reason the defenders of the upheaval hypothesis are con- 

 sistent with themselves in assigning the whole movement by which 



