1869.] Vesuvius. 193 



through the mass. The same is the case with the surfaces of lava 

 streams. 



Ascending oUiquely, from the direction of Bosco Tre Case and 

 Pompeii (writes Professor PhilHps, p. 191, March 1868), "we 

 reach a hroad hot region, in which are the principal fumaroli ; from 

 these quite lately — last night — lava was flowing freely, now barely 

 moving under a solid crust of pipy or cellular rock, deep sunk 

 below the rest of the surface, or merely glowing in deep cavities, 

 and yielding reluctantly to the pressure of a pole, which instantly 

 takes fire. Beneath our feet, however, for considerable spaces, the 

 ground is really fluid, only crusted over by the partially coherent 

 and very partially cooled scoriaceous blocks. Eemoving these, 

 where practicable, by the help of a pole, we see the sullen red lava 

 below. The quickness of the consolidation of the surface of lava 

 is remarkable ; we stand on the solid crust, over what remains of 

 yesterday's current, in a deep canal, which not long since ran full 

 of melted rock; on this very hot surface it is requisite to keep 

 in motion, if one has any regard for English shoes, but there is no 

 other inconvenience. 



" The au- is hot, somewhat hke that in the vicinity of iron- 

 furnaces — which seems, whether it be so or not, a little deficient in 

 oxygen, but no smell of sulphur, in any combination, and no 

 fumes of hydrochloric acid." 



The products issuing from the crater of Vesuvius are extremely 

 various, as also are the conditions under which they are ejected. 

 They may be roughly grouped under three heads, namely : 1st, 

 Gaseous ; 2nd, Liquid ; 3rd, Solid. Under the first we may in- 

 clude carbonic acid, sulphuretted hydrogen, free hydrogen, hydro- 

 chloric acid, and nitrogen ; under the second, liquid lava and 

 streams of boiling water often charged with fine mud ; the third 

 includes ashes, pumice, lapillo, puzzolana, and volcanic bombs. 



The gases referred to have been noticed in the exhalations from 

 the fumarole of the lava of Vesuvius, and escaping from the 

 numerous crevices in the sides of the vent after the termination of 

 an eruption. 



Lava-rocks, although differing more or less in mineral character, 

 are all found, upon analysis, to be composed of silicates of alumina, 

 or magnesia, with some protoxide of iron, potash or soda, and 

 lime. 



The texture of the great bulk of them is stony and granular, 

 often highly crystaUine ; the component crystals of felspar, augite, 

 or hornblende, mica, ohvine, and perhaps quartz, being, though 

 interlaced and interpenetrating, occasionally as distinct and large as 

 in granite.* 



* Scropc, chap, vii., pp. 110 unci 114. 



