SORBY — STRUCTURE OF CRYSTALS. 499 



Tigs. 53, 54, 55, 56. Fluid-cavities containing various crystals, in quartz re- 

 placing felspar, at Trevalgan, near St. Ives, Cornwall. 53, X 1600 j 

 54, 55, 56, X800. 



Plate XVIII. 



GLASS- AND STONE-CAVITIES IN THE MlNEEALS OE ERUPTED LAVAS, &C. 



Figs. 57, 58, 59, 60, 61, 62. Glass-cavities in the felspar of a porphyritic pitch- 

 stone in Arran. 57, X300; 58, X330; 59, X360; 60, Xl600j 

 61, X600; 62, X400. 



Fig. 63. A cavity of a different character in the felspar of a porphyritic pitch- 

 stone in Arran. X 600. 



Fig. 64. A glass-cavity in a prismatic crystal in a porphyritic pitchstone in 

 Arran. X300. 



Figs. 65, 66. Glass-cavities attached to crystals in the augite of the lava of 

 Vesuvius. 65, X500; 66, X400. 



Fig. 67. A stone-cavity attached to a crystal in the leucite of the lava of 

 Vesuvius. X300. 



Figs. 68, 69, 70. Stone- and glass-cavities in the leucite of the lava of Vesu- 

 vius. 68, X600; 69,70, X800. 



Figs. 71, 72. Stone-cavities in the felspar of the trachyte of Ponza. X 800. 



Figs. 73, 74. Grlass-cavities in the augite of a trappean rock at Balloch, Dum- 

 barton. 73, X450; 74, X600. 



Fig. 75. A glass-cavity in the augite of a trappean rock near Glasgow. X 600. 



Figs. 76, 77. Altered glass- or stone-cavities in the felspar of a porphyritic 

 greenstone at Arthur's Seat, near Edinburgh. 76, X400 ; 77, X200. 



Fluid-, Gas-, Vapour-, Stone- and Glass-cavities in Minerals formed 

 by the combined action of water and igneous fusion. 



Fig. 78. A fluid-cavity hi the calcite of a block ejected from Vesuvius. X 300. 



Figs. 79, 80, 83, 85. Fluid-cavities in the nepheline of a block ejected from 

 Vesuvius. X1000. 



Figs. 81, 82, 84. Fluid-cavities in the nepheline of a block ejected from Vesu- 

 vius, after having been heated to a more or less bright red heat. 



Figs. 86, 87. Gas-cavities in the nepheline of a block ejected from Vesuvius, hi 

 a natural state. 86, X1000; 87, X400. 



Fig. 88. A vapour-cavity in the nepheline of a block ejected from Vesuvius. 

 X 800. 



Plate XIX. 



Figs. 89, 90. Glass-cavities hi the nepheline of a block ejected from Vesuvius. 



89, X400; 90, X1200. 

 Fig. 91. The cavity, fig. 90, after having been heated to bright redness. 

 Figs. 92, 93. Fluid- cavities in the idocrase of a block ejected from Vesuvius. 



X1200. 

 Figs. 94, 95. Fluid- cavities in the felspar of a block ejected from Vesuvius. 



94, X800; 95, X1600. 

 Fig. 96. A fluid-cavity in the felspar of a block ejected from Vesuvius, with 



much enclosed vapour. X 500. 

 Fig. 97. A vapour-cavity in the felspar of a block ejected from Vesuvius, out of 



focus in the centre. X 500. 

 Fig. 98. A glass-cavity in the felspar of a block ejected from Vesuvius, out of 



focus in the centre. X 400. 

 Fig. 99. A stone-cavity in the felspar of a block ejected from Vesuvius, out of 



focus in the centre. X 500. 

 Figs. 100, 101. Fluid-cavities in the quartz of a trachyte from Ponza, 



100, X2000; 101, X800. 

 Fig. 102. A fluid-cavity in the quartz of a trachyte from Ponza, X1600, so 



placed that the bubble totally reflects the transmitted hght. 

 Fig. 103. A stone-cavity in the quartz of a trachyte from Ponza. X400. 



