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.
Notes
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.).