Expetlments on Whinjioite and Lava, 65 



yet entirely ignorant of the degree of force requlfite for this purpofe, we have no proof 

 thac it has been too weak. All the veins of Somma I examined were abfolutely compadt, 

 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 fur- 

 face, 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 

 degree of preflure ; for, in this refpeiSl, we have reafon to look for the greateft diverfity. 

 Some vertical ftreams muft have flowed while the mountain was yet low; others may have 

 found vent at a low level ; in both which cafes, the preflure would be feeble : whereas 

 other ftreams, communicating with elevated lateral eruptions, would fuftain, in their 

 lower parts, the full re-a£tion of deep columns of liquid lava, and may be cxpedled to ex- 

 hibit the effefts of great preflure. Should any future traveller be fortunate enough to 

 meet with a nodule of calcareous fpar in a lava, occupying the crevice of a mountain 

 formed by undoubted external eruptions, all that has been faid of the effeds of prefliire 

 would ceafe to be hypothetical, and this fundamental article of Dr. Hutton's theory would 

 be eftablifhed beyond difpute. 



I have now examined the relation between whinftone and lava in various points of view ; 

 and the refult of the inveftigation, by fhowing the intimate connexion between the two 

 clafles, tends ftrongly to confirm the ideas of Dr. Hutton. I flatter myfclf, likewife, that 

 the experiments, independently of the general views of geology, are of fome value, by ac- 

 counting for the ftony charaQer of lavas, and thus enabling us to difpenfe with the various 

 myftical fuppofitions which have of late perplexed the hiftory of volcanic phenomena. 



TABLE OF FUSIBILITIES. 



Vol. IV.— May 1800. 



K 



A Memoir 



