EXPERIMENT'S en PVHINStONE and LAVA. yj 



Jowed by a ftream fomewhat different, wliicPi had remain- 

 ed there on the cooUng of the whole. In one cafe, I found 

 lava on each fide, and in the middle tuffa, which is generally 

 fuppofed to have been enipted in the ftate of watery mud. In 

 another, the fub fiance in immediate contact with the moun- 

 tain 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 re- 

 femblance, in point of pofition, to veins of every defcription,^ 

 which, in all parts of the world, are found penetrating the 

 ftrata, and which, according 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 expedl that, if examined 

 with particular care, their agreement will be found complete. Of 

 this, however, we muft not form too fanguine expedlations ; for 

 tliough the vertical lavas of Somma have undoubtedly fuftained 

 the prelFure 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 entirely ignorant of 

 the degree of force requifite for this purpofe, we have no proof 

 that it has been too weak. All the veins of Somma I examined 

 were abfolutely compadl, except one which was full of pores. I 

 am unable to determine whether this was the real porofity of a 

 lava, or whether, as in our whins, it arofe from the removal, 

 near the furface, of nodules of calcareous fpar. Granting that 

 thefe pores were real air holes, the circumftance was peculiar to 

 that fingle ftream, and may have been owing to an inferior de- 

 gree of prefTure ; for, in this refped, we have reafon to look for 



Part I. K the 



