SOME ANCIENT AND MODEEN VOLCANIC KOCKS. 397 



but generally united, like strings of beads, parallel to the crystal 

 sides. He also describes it as specially characteristic of leucite 

 crystals that they are surrounded outside by a close-lying tangential 

 augite-microlitic circle. 



3. Leucttic Basalt. Near Torre del Greco. 



This rock, which is part of the lava-flow of 1794, exhibits a 

 well-marked columnar structure. It is compact, contains larger 

 leucite crystals than the last, and they are further apart, while the 

 augite is in greater abundance, and the crystals usually larger. 



The general microscopic character of this lava is shown in figs. 

 4 and 5, the latter representing a portion of the former more highly 

 magnified. The light brown is augite, the highest example in the 

 figure being a group of crystalline grains, which come out distinct 

 from one another, in colour, under polarized light, and are represented 

 in their natural size just below the figure. The white meshes 

 are leucite ; and the base consists of numerous magnetite crystals, 

 acicular prisms of triclinic felspar, and a greenish and brown dif- 

 fused mineral, probably in great measure a product of alteration. 

 Every here and there larger prismatic crystals of triclinic felspar 

 occur imbedded in the base. 



The chief points to be noticed are the following : — The leucite 

 crystals stand out more apart from each other than in the example 

 of the 1631 lava, and have much more of the base between them. 

 In no one instance have I observed the circular series of stone- 

 cavities so common within the closely croivded crystals of the last 

 specimen ; but the crystals contain many acicular prisms and some 

 grains of magnetite. Glass- and stone- cavities are by no means so 

 abundant as in the former example. There are a few instances of 

 what must be either gas- or vapour- cavities, or circular glass-cavi- 

 ties, converted into stone-cavities, all but the central point ; in fig. 4 

 is one of these bubble-like bodies, within a leucite crystal, next to the 

 augite, at the south-western corner. Many of the leucite crystals 

 also show, under crossed prisms, the finely striated structure (fig. 32), 

 which will be noticed again in describing the next specimen. 



The augite crystals present many glass-cavities, some of which 

 contain magnetite grains, as is the case with one in fig. 33 ; and 

 others, having the crystalline form of augite, give rise to a class of 

 negative crystals (fig. 5). The augite also encloses long prismatic 

 crystals (fig. 33), and some shorter and thicker ones. In one 

 instance a group of augite crystals seems to have been surrounded 

 by leucite. In some cases, however, it would seem as if the 

 augite crystals had been broken, and the leucite had crystallized 

 around and among the broken pieces; thus, in fig. 4, the piece of 

 augite on the western edge of the disk was probably (the whole 

 crystal being seen) the continuation of the lower portion ; but it 

 seems to have been broken away, and the leucite to have solidified 

 between the severed fragments. In the lower part of this same 

 augite crystal (fig. 33) there occur a great many glass-cavities, 

 some large and filled with leucite, but many very small, arranged 



