MODIFIED by COMPRESSION. 167 



pies. And it thus appears, that whinftone pouenes all the pro- 

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

 lava. 



This collection is curioufly illuflrated by an intermediate 

 cafe between the remits of external and internal fire, difplayed 

 in an actual feclion of the ancient part of Vefuvius, which oc- 

 curs in the Mountain of Somma mentioned above. I formerly 

 defcribed this fcene in my paper on Whinftone and Lava; and 

 I muft beg leave once more to prefs it upon the notice of the 

 public, as affording to future travellers a moft interefting field 

 of geological inquiry. 



The fe&ion is feen in the bare vertical cliff, feveral hundred 

 feet in height, which Somma prefents to the view from the 

 little valley, in form of a crefcent, which lies between Somma 

 and the interior cone of Vefuvius, called the Atrio del Cavallo. 

 (Fig. 42. reprefents this fcene, done from the recollection of 

 what I faw in 1785. abc is the interior cone of Vefuvius ; 

 dfg the mountain of Somma; and c de the Atrio del Cavallo). 

 By means of this cliff (fd in figure 42. and which is repre- 

 fented feparately in fig. 44.), we fee the internal ftructure of 

 the mountain, compofed of thick beds {k k) of loofe fcoria, 

 which have fallen in mowers ; between which thin but firm 

 dreams (m m) of lava are interpofed, which have flowed down 

 the outward conical fides of the mountain. (Fig. 43. is an ideal 

 feciron of Vefuvius and Somma, through the axis of the cones, 

 fhewing the manner in which the beds of fcoria and of lava 

 lie upon each other; the extremities of which beds are {ten. 

 edgewife in the cliff at m m and k k, fig. 42, 43, and* 44.). 



This aflemblage of fcoria and lava is traverfed abruptly and 

 vertically, by ftreams of folid lava {n n, fig, 44.), reaching 

 from top to bottom of the cliff. Thefe laft I conceive to have 

 flowed in rents of the ancient mountain, which rents had acled 



as. 



