BY COMPRESSION. 211 



and wliicli cease as soon as the lava has found a vent. It 

 seems but reasonable to ascribe like effects to like causes, and 

 to believe that the earthquakes which frequently desolate coun- 

 tries riot externally volcanic, likewise indicate the protrusion 

 from below of matter in liquid fusion, penetrating the mass of ' 



rock. 



The injection of a whinstone-dike into a frail mass of shale The varying- 

 and sandstone, must have produced the same effects upon itpt^'ed^bv^olT" 

 that the lava has just been stated to produce on the loose beds of lavas to the 

 volcanic scoria. One stream of liquid whin, having flowed ^^^^^^1^^°^ "^^'^'^ 

 into such an assemblage, must have given it great additional variety in the 

 weight and strength : so that a second stream coming like the P'^^'^omena, 

 first, would be opposed by a mass, the laceration of which would 

 produce an earthquake, if it were overcome; or by which, if it 

 resisted, the liquid matter would be compelled to penetrate some 

 weaker mass, perhaps at a gieat distance from the first. The 

 internal fire being thus compelled perpetually to change the scene 

 of its action, its influence might be carried to an indefinite ex- 

 tent : So that the intermittance in point of time, as well as the 

 versatility in point of place, aheady remarked as common to the 

 Huttonian and Volcanic fires, are accounted for on our princi- 

 ples. And it thus appears, that whinstone possesses all the pro- 

 perties which we are led by theory to ascribe to an internal 

 kva. 



This connection is curiously illustrated by an intermediate 

 case between the results of external and internal fire, displayed 

 in an actual section of the ancient part of Vesuvius, which oc- 

 curs in the Mountain of Somma mentioned above. I formerly 

 describcd-this scene in my paper on Whinstone and Lava ; and 

 I must beg leave once more to press it upon the notice of the 

 public, as affording to future travellere a most interesting field 

 of geological inquiry. 



The section is seen in the bare vertical cliff, several hundred Remarkable 

 feet in height, which Somma presents to the view from the f^'^**^''''^^.'^^** 

 little valley, in form of a crescent, which lies between Somma Cliff of Sanuaa. 

 and the interior cone of Vesuvius, called the Atrio del Cavalh. 

 (-Fig. 42. represents this scene, done from the recollection of 

 what I saw in 1785. a b c is- the interior cone of Vesuvius ; 

 dfg the mountain of Somma ; and c J e the Atiio del CavelloJ. 

 By means of this cliff (/d in Fig. '42. and which is represented 

 E e 2 sepa* 



