AND THEIR MEEETING WITH GRANITE. 87 



paper " On the Effects of Heat modified by Compression.*' 

 This scene, as viewed in the Atrio del Cavallo, (Plate V. 

 volume VI. figs. 41, 42, 43, and 44.), exhibits in nature a com- 

 plete section of the old volcano of Vesuvius, now called Som- 

 ma. The mountain is there seen to be composed of a succes-^ 

 sion of beds of lava and of cinders, the lava occupying only a 

 fourth or a fifth part of the mass, wliich is traversed vertically, 

 but irregularly, by numerous rents filled with solid lava;, 

 these rents, as 1 have endeavoured to prove, having undoubt- 

 edly served as the pipes through which lateral eruptions have 

 been discharged. 



Each of these rents would continue open during the course 

 of the particular eruption by which it was formed, and the la- 

 va would flow freely through it ; but when the eruptive im- 

 pulse ceased, it would remain full of the liquid lava, which 

 would congeal, so as to leave the rent, as it now appears, 

 completely filled with hard and solid rock. This new sub- 

 stance welding itself firmly to the extremities of the beds 

 of lava which had been broken across, would bind them toge- 

 ther into a species of net-work, and thus the injury done to 

 the mountain by the formation of the rent, would be repaired, 

 and much more than repaired ; so that when a new eruptioa. 

 was directed to the same quarter, it would be less able to pe- 

 netrate than before, and the eruption would be restrained till 

 a fresh rent was effected in some other part of the mountain. 

 A new eruption must thus, in every case, be an act of vio- 

 lence ; and we see how a lateral emption may be followed, as- 

 frequently happens, by a discharge of lava from the summit 

 of the mountain, which could not have taken place, had the 

 first lava continued fluid, since it would never have ceased in: 

 that case to flow through the lowest aperture, whereas, in 



consequence 



