er of these the so-called granites of the
Mourne Mountains, of Arran Island, and of Skye are to be referred.
Granite is a rock which has been intruded in a molten condition
amongst the deep-seated parts of the crust, and has consolidated
under great pressure in presence of aqueous vapour and with extreme
slowness, resulting in the formation of a rock which is largely
crystalline-granular. Its presence at the surface is due to denudation
of the masses by which it was originally overspread.

[Illustration: Plate I.]

EXPLANATION OF PLATE I.

MAGNIFIED SECTIONS OF VESUVIAN MINERALS.

Fig. 1. Section of leucite crystal from the lava of 1868, with
fluid cavities. Mag., 350 diams.

" 2, 3, 4, and 5. Sections of nepheline crystals from the lava of
1767, 1834, and 1854.

" 6. Section of sodalite crystal from the lava of 1794, with
belonites and crystals of magnetite enclosed.

" 7, 8, 9. Crystals of leucite with microliths and cavities
darkened by magnetite dust; also, containing crystals of
magnetite.

" 10. Group of leucite crystals of irregular form from the lava of
1855, congregated around a nucleus of crystals of plagioclase
and magnetite.

[Illustration: Plate II.]

EXPLANATION OF PLATE II.

MAGNIFIED SECTIONS OF VESUVIAN MINERALS.

Fig. 1. Section of augite crystal from the lava of 1794, with
numerous gas cells and delicately banded walls. The interior
contains two long prisms, probably of apatite.

" 2. Crystal of augite with banded walls, and indented by leucite
crystals, from the lava of 1794. Mag., 40 diams.

" 3, 4, 5. Sections of augite crystals from the lavas of 1794 and
1820.

" 6. Group of augite crystals from the lava of 1835.

" 7. Ditto from the lava of 1822, with encluded mica-flake (_a_)
and portion of the glass paste, or ground-mass, of the rock
(_b_), containing microliths and grains of magnetite.

Fig. 8. Two crystals of olivine from the lava of 1855; they are
intersected on one side by the plane of the thin section, and
are remarkable for showing lines of gas cells, and bands of
growth sometimes cellular. Mag., 40 diams.

" 9. Section of rock-crystal (quartz), with double terminal
pyramids, from the lava of 1850.

" 10. Twin crystal of sanidine fro