SOME ANCIENT AND MODERN VOLCA.NIC ROCKS. 



395 



crystal a bubble occurs independently of a cavity (fig. 30) ; this is 

 either a gas-cavity, or a circular stone-cavity, with glass in the centre 

 forming a point of light. The presence of a decided bubble within 

 a glass- and stone-cavity is of considerable significance, because, as 

 Sorby remarks (p. 479), " nothing but igneous fusion could so 

 liquefy the enclosed glass that perfectly spherical bubbles could be 

 produced." 



The frequent circular arrangement of the stone-cavities, as noticed 

 above, would seem to indicate that solidification had taken place 

 more rapidly near the circumference of the crystal than at the 

 centre, where such cavities are much more seldom met with*. 

 Stone -cavities are frequently seen to be attached to acicular crystals ; 

 of this Sorby gives an example ; and in all the cases I have ob- 

 served the cavity is of an oval form, with its longest diameter in the 

 direction of the length of the crystal (figs. 23 and 27), clearly 

 showing that the original cavity was drawn out along the line of 

 the crystal before the solidification of its contents, and whilst the 

 surrounding leucite was in a more or less viscid state. 



Figs. 23-31. — Leucite crystals containing microlites, glass-, and stone- 

 cavities, and magnetite grains. Fig. 23 a represents the average 

 size of these leucite crystals. 



23a. 



23. 



'^ 



24. 



(30 



28. 



Fig. 32. — Striated structure 

 of leucite. Polarized 

 light. 



A: 



/.\ 



29. 



30. 



20. 



31. 



27. 



Fig. 33. 



Part of the aur/ite crystal 

 in fig. 5 (Plate XVIL), more 

 highly magnified. 



Acicular crystals also frequently occur within the leucite inde- 

 pendently of glass- or stone-cavities, as in figs. 28 and 31. In 

 fig. 29, along with some small acicular crystals and minute stone- 

 cavities, is a grain of magnetite ; and the almost triangular dark spot 

 in fig. 27 is probably magnetite also. 



The order in which the constituent minerals of this rock seem to 

 have crystallized out, as mentioned above, appears at first sigh 



* Cavities, whether stone, glass, or liquid, are usually more numerous the 

 more rapid the process of solidification has been. 



