Volcanoes: Past and Present Part 17

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 magnet.i.te enclosed.

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

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

[Ill.u.s.tration: 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 apat.i.te.

" 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 gla.s.s paste, or ground-ma.s.s, of the rock (_b_), containing microliths and grains of magnet.i.te.

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 from the lava of 1858. Mag., 40 diams.

" 11, 12, 13. Sections of plagioclase crystals (probably labradorite) from the lava of 1855. Mag., 100 diams.

" 14. Section of olivine crystal from the lava of 1631--imperfectly formed. Mag., 30 diams.

" 15. Section of mica-flake from the lava of 1822. Mag., 30 diams.

[Ill.u.s.tration: Plate III.]

EXPLANATION OF PLATE III.

MAGNIFIED SECTIONS OF VOLCANIC ROCKS.

1. Diorite d.y.k.e, traversing a.s.synt limestone, North Highlands.

2. Basalt from upper beds, near Giant's Causeway, County Antrim.

3. Hornblende-hypersthene-augite Andesite, from Pichupichu, Andes.

4. Augite-Andesite from Pichupichu, Andes.

5. Olivine dolerite, with hornblende and biot.i.te, Madagascar.

6. Leucite basalt, with mellilite, Capo di Bove, Italy.

[Ill.u.s.tration: Plate IV.]

EXPLANATION OF PLATE IV.

MAGNIFIED SECTIONS OF VOLCANIC ROCKS.

1. Vesuvian lava, gla.s.s paste with numerous crystals of leucite; others of augite and nepheline porphyritically developed; also small grains of magnet.i.te.

2. Vesuvian lava, gla.s.s paste with numerous crystals of leucite; others of olivine, hornblende, and sanidine, porphyritically developed; small grains of magnet.i.te.

3. Trachyte from Hungary; felsitic paste with crystals of hornblende and sanidine, and a little magnet.i.te.

4. Gabbro, from Carlingford Hill, Ireland, consisting of anorthite, augite, a little olivine, and magnet.i.te.

5. Dolerite, from old volcanic neck, Scalot Hill, near Lame, consisting of labradorite, augite, olivine, and magnet.i.te.

6. Dolerite, Ballintoy, County Antrim, showing ophetic structure, consisting of augite, labradorite, and magnet.i.te.

[1] Mr. S. Allport has discovered this in the rock called the "Wolf Rock" off the coast of Cornwall. The most important work on basalt is that by F. Zirkel, _Unters. uber mikros. Zusammensetzung und Structur der Basaltgesteine_. Bonn (1870).

[2] Zirkel, _Die mikroskopische Beschaffenheit der Mineralien und Gesteine_, p. 153. Leipsig (1873).

[3] Zirkel, _Petrog._, i. 578; B. von Cotta, p. 178 (Eng. Trans.).