.NOME I. COMPACT LAVA. 



pure melted felsite. Dolomieu specially observes 

 that the tint is original, and not derived from sul- 

 phurous vapours*. Even earthy lavas and basalt 

 may be found of a white colour ; but this always 

 arises from the action of vapours. White lava is 

 found in the little isle of Ischiaf. 



Micronome 3. Brown compact lava. 

 This colour may arise from the iron mingled in 

 red felsite. 



HYPONOME III. PORPHYRITfC LAVA. 



As both the substances most general in lavas, 

 namely, siderite and felspar, also constitute ge- 

 nuine porphyry, it is naturally to be expected 

 that lava should sometimes assume this structure. 

 The ingenious observer of Etna gives the follow- 

 ing account^. 



" I denominate all those lavas porphyritic, 

 which present crystals of felspar, when those 

 crystals are of a different colour from the base 

 which contains them, and from spots in it. 



" This species is most common : it in itself con- 

 stitutes more than half of the compact lavas of 



* Etna, l6l. 



f Ponces, p. 71, and 1(X). 



J Dolomieu, 212. 



