394 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



The lava, though often very hard and compact, is frequently full 

 of small vesicles ; the leucite crystals appear as a number of small 

 white specks, and the augite is dark-green, while both are im- 

 bedded in a leucitic and augitic base. 



Microscopic examination (Plate XVII. fig. 3). — The general appear- 

 ance of this rock, when viewed in a thin slice under the microscope, 

 is that of an open mesh-work (fig. 3), in which the meshes are 

 formed by the transparent leucite, and the threads by minute crystals 

 of magnetite, small prisms of triclinic felspar, and augite. A few 

 flakes of brown magnesia-mica and some large crystals of augite are 

 scattered about. With polarized light the leucite is black under 

 crossed prisms, while the other ingredients, except the magnetite, 

 show more or less of colour. 



In fig. 3 the following points should be specially noted. 1. The 

 flake of mica seems broken in two, augite now separating the two 

 parts. 2. The magnetite grains are thickly clustered together around 

 the mica. 3. The large crystal of augite contains several cavities 

 filled with leucite, in one of which is a small magnetite crystal.* 4. 

 Some of the small felspar prisms shoot out into the leucite meshes. 



From which one would infer: — 1. That the magnetite was 

 the first to crystallize, or was already formed when the lava was 

 erupted. 2. Before the other minerals crystallized, the mica plate, 

 shown in the figure, was probably cracked; and it seems likely that 

 augite, solidifying within the crack, separated the two pieces, and 

 caused them to push the magnetite grains in front of them *. 3. The 

 larger augite crystals in their formation caught up some of the viscid 

 leucite, and with it an occasional magnetite grain. 4. The leucite 

 crystallized out, enclosing small felspar prisms and a few magnetite 

 crystals. The crystalline form of the leucite is seldom seen to 

 perfection, owing partly, no doubt, to the interference of the crystals 

 one with another. 



In the crystals of leucite there occur a great many stone-cavities, 

 usually of a more or less circular form. The arrangement of these 

 cavities is especially striking. In a very large proportion of the 

 crystals they form a circle a short distance within the circum- 

 ference, as seen in figs. 24, 26, 27 (fig. 23 a represents the average 

 size of these leucite crystals). In most cases the cavities are com- 

 pletely crystalline and dark ; but some of the larger ones are only 

 partially so, thus forming a combined glass- and stone-cavity, as 

 figured by Sorby in his paper on " The Structure of Crystals " 

 (plate xviii. fig. .68) t. The same author says, "in no case have 

 I seen decided bubbles in the cavities in leucite ;" in one instance, 

 however, I have found a bubble occupying fully one third of a 

 compound glass- and stone-cavity, as seen in fig. 25. In another 



* Allporthas found felspar crystals in the Pitclistone of Planitz, with opaque 

 particles crowded round their sides ; and he remarks, " this clearly indicates that 

 during the formation of the crystals the matrix was in a viscid but not in a fluid 

 state ; for had the particles been quite free to move, there would have been no 

 crowding" ("The Microscopical Examination of Rocks and Minerals," Monthly 

 Microscopic Journal, August 1870). 



t Quart. Journ. Geol. Soc. vol. xiv. p. 478. 



