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. [Illustration: Plate III.] EXPLANATION OF PLATE III. MAGNIFIED SECTIONS OF VOLCANIC ROCKS. 1. Diorite dyke, traversing Assynt 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 biotite, Madagascar. 6. Leucite basalt, with mellilite, Capo di Bove, Italy. [Illustration: Plate IV.] EXPLANATION OF PLATE IV. MAGNIFIED SECTIONS OF VOLCANIC ROCKS. 1. Vesuvian lava, glass paste with numerous crystals of leucite; others of augite and nepheline porphyritically developed; also small grains of magnetite. 2. Vesuvian lava, glass paste with numerous crystals of leucite; others of olivine, hornblende, and sanidine, porphyritically developed; small grains of magnetite. 3. Trachyte from Hungary; felsitic paste with crystals of hornblende and sanidine, and a little magnetite. 4. Gabbro, from Carlingford Hill, Ireland, consisting of anorthite, augite, a little olivine, and magnetite. 5. Dolerite, from old volcanic neck, Scalot Hill, near Lame, consisting of labradorite, augite, olivine, and magnetite. 6. Dolerite, Ballintoy, County Antrim, showing ophetic structure, consisting of augite, labradorite, and magnetite. [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. über 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.). INDEX. INDEX. Abyssinian table-lands, 190 _et seq._ Albano, Lake, 89 America, volcanic regions of North, 136 _et seq._; of Western, 144 Andes, 18, 27, 227, 254 Andesite, 263 Antrim, 154 _et seq._ Arabia, dormant volcanoes of, 126-135 Arabian desert, 134 Archibald, C. D., 213 Arizona, volcanoes of, 137 Argyll, Duke of, 173