and the allied Rocks. 51 



free quartz of granite. Other varieties of mica contain mag- 

 nesia, which would go towards the formation of some mineral 

 of the magnesian series. It is possible that trachytes and 

 porphyry have thus been made from granite; but trap rocks 

 could not have been so derived, as they contain from 10 to 

 25 per cent, less of silica. 



Again, hornblende and augite are so nearly related, that 

 they have been considered by Rose the same mineral, the dif- 

 ferent circumstances attending the cooling giving rise to the 

 few peculiarities presented. There can be no difficulty there- 

 fore in deriving augite by fusion from hornblende rocks. This 

 moreover has been actually confirmed by experiment. 



Augite, by giving up half of its silica, and receiving addi- 

 tional magnesia in place of its lime, is reduced to chrysolite*. 

 The Gehlenite, nepheline, anorthite and meionite of Vesuvius, 

 contain, like scapolite, only 40 to 4-5 per cent, of silica and a 

 large proportion of lime, and it is no improbable supposition, 

 judging from the small amount of silica, and from the lime 

 present, that scapolite rock, or rather limestones containing 

 scapolite, may have contributed in part towards the lavas of 

 that region. The ejections of unaltered granular limestones, 

 and many mineral species pertaining to such beds, strongly 

 support this view; and it is no less sustained by the fact, that 

 in the Vesuvian basalts, Labradorite, which includes lime in- 

 stead of the alkalies, replaces common felspar. The original 

 felspar seems to have given way to leucite and Labradorite f. 



An important source of new combinations is found in the 

 sea-water which gains access to the fires of volcanoes. The 

 decomposition which takes place eliminates muriatic acid, so 

 often detected among volcanic vapours; but the soda and 

 other fixed constituents remain, to enter into combination 

 with some of the ingredients in fusion. Is not this one source 

 of the soda forming the soda felspar, or albite, and of the 

 muriatic acid and soda in sodalite? Phosphates have been 

 long known to occur occasionally in volcanic rocks, and lately 

 phosphoric acid has been proved to be generally common in 

 small quantities. Sea-water is also a very probable source of 

 this ingredient, as has been shown by late analyses of the same 

 by Dr. Jackson. 



* The formula of augite is R 3 Si 2 ; that of chrysolite, R 3 Si. 



t Using R for the bases and Si for silica, the formula of leucite is R Si T ; 

 that of common felspar, RSi 2 ; that of Labradorite, RSi. From this, it 

 appears that felspar may be reduced to leucite by giving up one-third of 

 its silica, the bases being the same in the two; and with this excess and 

 other silica combining with the lime at hand, Labradorite might be formed. 



E2 



