64 Experiments m Wh'injlone and Lava: 



raneous produflions of heat. I now tliink, that, though we judged rightly in believing 

 thofe lavas to have flowed in crevices, we were miftaken as to their diredlion ; for inftead 

 of flowing downwards, I am convinced they have flowed upwards, and that the crevices 

 have performed the office of pipes, through which lateral explofions have found a vent. 

 This will appear in the higheft degree probable, when we attend to the known hiftory of 

 volcanic eruptions. It generally happens, that the lava begins to flow from the fummit, in 

 confequence of the crater being filled with liquid matter up to the brim. At that moment 

 the bads of the mountain muft be prefied outwards by a very great hydroftatical force, 

 equal to the weight of a column of liquid lava as high as the mountain itfelf. It is natural 

 then to expeft, that this prefiure, affifted by ftrong percuffions of explofion, fliould lace- 

 rate the body of the hill, and form great rents. The lava, urged upwards by the fame 

 preflure, would flow through tbefe rents, and emerge at the furface with violence. The 

 difcharge would continue through this channel till the propelling force had ceafed, when 

 the rents would be left full of lava; which, cooling in that pofition, would produce ver- 

 tical lavas, fuch as thofe of Somma. This fuppofition is confirmed by various phenomena: 

 the lava ceafes to flow from the crater as foon as a lateral eruption has begun ; when it 

 rulhes with fuch violence from the fide of the mountain as to fly to a great height into the 

 air, like z Jet efeau; and it often makes its appearance, in the fame inftant, at various 

 mouths, which are not fcattered at random, but placed in one continued line, indicating 

 the difcharge from a rent. Some circumftances likewife, which I obferved on a clofe exa- 

 mination of the vertical lavas, indicate that the crevices had performed the office of pipes. 

 Frequently the fubftance at the middle differs from that on each fide, whilft the fides re- 

 femble each other exactly. I explain this, by fuppofing that the lava, which had firll 

 flowed through the pipe, and had coated its fides with folid matter, had been followed by 

 a ftream fomewhat different, which had remained there on the cooling of the whole. In 

 one cafe, I found lava on each fide, and in the middle tuS"a, which is generally fuppofed 

 to have been erupted in the ftate of watery mud. In another, the fubftance in immediate 

 contaft with the mountain is vitreous, the reft being common lava. This is fully ex- 

 plained by our experiments, if we fuppofe the ftream to have flowed into a cold crevice. 



To apply thefe obfervations to the general hiftory of the globe: it is evident that the, 

 vertical lavas bear the clofeft refemblance, in point of pofition, to veins of every defcrip- 

 tion, which, in all parts of the world, are found pftietrating the ftrata, and which, ac- 

 cording to Dr. Hutton, have flowed by means of fubterraneous heat. The veins, or dikes, 

 (as they are called), of whinftone, which fo commonly occur in this country, differ from 

 them in no circumftance which I had the means of obferving. It is therefore natural to 

 cxpeft that, if examined with particular care, their agreement will be found complete. 

 Of this, however, we muft not form too fanguine expeftations ; for though the vertical 

 lavas of Somma have undoubtedly fuftained the preffure of a great fuperincumbent mafs, 

 we have no proof that this force was fufficiently ftrong, as Dr. Hutton fuppofes was the 

 cafe in whinftone, to reprefs the volatility of carbonic acid. On the other hand, as we are 



yet 



