180 Accoimt of the Eruption of Vesuvius, 



a more or less pearly lustre, apparently in proportion to the 

 greater or less degree of torrefaction they have endured. The 

 fusion of the Icucites seems to be the cause of this appearance. 

 In some specimens this process has been carried to such ex- 

 tremity that a portion of the lava has run into a black glass, 

 which fairly merits the name of Leitcitic Obsidian. In colour, 

 fracture, and transparency, this substance resembles the common 

 trachytic obsidian of Lipari, but differs from it in melting 

 before the blowpipe into a black glass, while the obsidian of 

 Lipari is well known to produce one of a greyish-whitfr 

 colour. 



But this is not the only alteration produced on these erratic 

 blocks of lava, by their re-exposure to the intense action of the 

 volcanic furnace. In some cellular specimens, the cavities are 

 thickly lined with crystals of specular iron, and of various 

 other minerals, hitherto undescribed, if not unknown. Amongst 

 these, the most remarkable are delicate capillary crystals, which 

 are found by the lens to be hexagonal prisms, hollow within, 

 formed by the lateral junction of six long rectangular plates. 

 They are either white, or of a light flesh-red colour, and occupy 

 cavities which seem to have been produced by the total or par- 

 tial disappearance of the larger crystals of leucite. Acicular 

 radiated mcsotype occurs in the same mannei- ; as well as bril- 

 liant crystals in rhomboidal dodecahedrons, of a dark-green 

 colour. These new crystalline minerals, thus, to all appear- 

 ance, created out of the elements of a lava composed simply of 

 leucite and augite, during its re-exposure, under peculiar cir- 

 cumstances, to the action of volcanic heat, may be expected to 

 throw a useful light on the origin of the numerous and proble- 

 matic minerals occurring in those erratic blocks of crystalline 

 limestone, Sfc. Sfc, of the Monte Somma, which appear to 

 have undergone a similar process during the activity of that 

 ancient and enormous volcano ; and a stronger degree of pro- 

 bability is thus added to the opinion, by which these blocks of 

 limestone, with their accompanying mica, augite, garnet, vesu- 

 vian, nepheline, Sfc. ^c, are supposed to be, not unaltered 

 fragments of primitive rocks, but portions, perhaps, of the cal- 



