194 Vesuvius. [April, 



Mr. Lobley furnishes tis in his Appendix * with a hst of the 

 minerals found about Vesuvius and Somma on the authority of 

 Professor Scacchi. Of these thirty-three occur in Somma, four 

 in Vesu^dus, four others are common to the lavas of both, and two 

 were obtained from the fumaroles. 



This great disparity in the relative numbers of mineral species 

 met with in Somma and Vesuvius has a most important bearing on 

 their origin in volcanic rocks, and confirms the opinion generally 

 held by mineralogists, that a very large proportion of them owe 

 their origin to a slow process of disintegration and re-combination 

 of the constituents of volcanic tuffs, and lavas, in which rain-water 

 acts a most important part as carrier. 



The above opinion chiefly refers to those compound minerals 

 which form the great bulk of Professor Scacchi's list, and does not 

 in the least mvalidate the observations of Bischoft', Darwin, Forbes, 

 Monticelli, and Covelli, as to the formation of crystals of oli^^Ile, 

 quartz, albites, leucite, &c., previous to the emission of lava, or 

 whilst flowing at an exceedingly high temperature. 



The most glassy and semitranslucent varieties of lava are known 

 by the name of " obsidian." It does not occur m this condition 

 around Vesuvius, but is abundant in the Lipari Islands, Mount 

 Ararat, &c. 



Large tracts in Mexico (called 3Ialpms) are covered with 

 " obsidian," and of it (as from the chalk-flint in Western Europe) 

 the worshippers of the sun manufactured their knives, &c., which 

 were still in use at tht» time of the Spanish conquest. 



So long ago as 1825, Mr. Scrope arrived at the conclusion that 

 the mobility of the solid component particles of liquid lava was not 

 due to the mass being in a state of molecular fusion, in which 

 condition it never occurs — subaerially, — but to the presence of an 

 interstitial fluid disseminated through the mass, and that this fluid 

 was water in a highly comminuted condition. This conclusion he 

 seems to have arrived at from observing that the mcandescent lava 

 at the moment of its exposiure and in the act of consohdation always 

 gave off abundance of steam. 



This hydro-igneous theory of lava has since been remarkably 

 confirmed by the microscopic uivestigations of ]\Ir. Sorby.j 



The hard materials ejected from the crater are oidy diverse con- 

 ditions of lava. Thus, when by the violent ebullition in the crater 

 a more liquid portion of lava is shot up into the air, it assumes a 

 globular or pearshaped figure, and is known as a volcanic " bomb," 

 Other fragments of lava projected at a slightly lower temperature 

 possess ragged, tattered shapes, and are full of vesicles ; the heavier 

 are called " scoria:^," the lighter " pumice." 



* ' Volonnnoa of Ci^ntnil France,' pp. .'J4 and .')5. 

 t '<iunrt. Journ. Cicol. Soc.,' vol. xiv., pp. 4.53-500. 



