396 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OE 



perhaps somewhat anomalous, since their fusibility before the blow- 

 pipe stands in the following order : — 



Leucite, infusible. 



Magnetite, fuses with extreme difficulty. 



Magnesia-mica, difficultly fusible. 



Felspar, fusible, but often with difficulty. 



Augite, generally fusible. 

 Nevertheless it seems, from the various facts noticed in the pre- 

 ceding paragraphs, that the four last-named minerals were held in 

 solution by leucite in a state of fusion ; and that instead of this 

 mineral crystallizing out first, it deposited in succession the mag- 

 netite, the mica, the felspar, and the augite, and last of all probably 

 solidified quickly, enclosing within its crystals glass- and stone- 

 cavities, and magnetite and felspar crystals. Sorby, in speaking of 

 the analogy between glass-cavities and fluid-cavities, says *, " their 

 peculiar characters can be most perfectly explained, if we suppose 

 that the glassy base, when in a state of fusion, acted like a solvent 

 liquid and dissolved various mineral substances, which were de- 

 posited on cooling in precisely the same manner as crystals are 

 deposited on the cooling of a saturated aqueous solution. There is 

 therefore, in my opinion, no more necessary connexion between the 

 temperature at which the crystals were deposited from this glassy 

 solvent and their own fusing-point when heated alone, than between 

 the temperature at which crystals are deposited from solution in 

 water and their own fusing-point, even if they be fusible. In both 

 cases the only necessary connexion is, that the crystals could not be 

 deposited in a solid form, except at a lower temperature than that 

 at which they become liquid ; but it might be any heat less than that 

 high enough to cause the glassy solvent to be sufficiently fluid." 



Might not the circular distribution of the stone- cavities, as 

 noticed of such frequent occurrence in this specimen, indicate that, 

 after the leucitic glassy solvent had deposited the other minerals, 

 it began to crystallize at numerous points, slowly at first, thus 

 pushing outwards and away from these centres most of the pre- 

 viously formed minerals, until, as the growing leucitic crystals more 

 nearly approached each other, the solidification became more rapid, 

 and, in consequence, more cavities and particles were enclosed near 

 their outer boundaries than at the centre ? 



Zirkel describes f some leucite crystals from Capo di Bove, in 

 which there is an outer circle of opaque grains, and an inner one 

 of clear glass with bubbles. On p. 149 he mentions the various 

 bodies enclosed in leucite : — little needles or grains of augite ; colour- 

 less felspar microlites ; roundish or egg-shaped blackish or brownish 

 transparent grains which, for the most part, belong to a half glassy 

 substance ; pure glass-cavities ; and opaque particles of magnetite. 

 Eosenbusch £ mentions cases of glass-cavities in leucite with inter- 

 secting bubbles, and remarks that gas-pores are seldom found singly, 



* " Structure of Crystals," Quart. Journ. Geol. Soc. vol. xiv. p. 477. 

 t Mikroskopische Beschaffenheit, p. 75. 

 \ Mikroskopische Physiographic, p. 192. 



