CHAPTER XII. THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC ACTION 331 INDEX 371 ILLUSTRATIONS. Sections of igneous rocks illustrating the passage from the glassy to the crystalline structure _Frontispiece_ Fig. Page 1. Stromboli, viewed from the north-west, April 1874 _to face p._ 10 2. Map of the Island of Stromboli 11 3. Section through the Island of Stromboli from north-west to south-east 13 4. The crater of Stromboli as viewed from the side of the Sciarra during an eruption on the morning of April 24, 1874. 14 5. Vesuvius in eruption, as seen from Naples, April 26, 1872. (_From a photograph_) _to face p._ 24 6. View of Vulcano, with Vulcanello in the foreground--taken from the south end of the Island of Lipari 43 7. Minute cavities, containing liquids, in the crystals of rocks. (_After Zirkel_) _to face p._ 60 8. Minute liquid-cavity in a crystal, with a moving bubble. (_After Hartley_) 63 9. Cavity in crystal, containing carbonic-acid gas at a temperature of 86° F., and passing from the liquid to the gaseous condition. (_After Hartley_) 64 10. Monte Nuovo (440 ft high) on the shores of the Bay of Naples. (_After Scrope_) 76 11. Map of the district around Naples, showing Monte Nuovo and the surrounding volcanoes of older date 78 12. Outlines of the summit of Vesuvius during the eruption of 1767. (_After Sir W. Hamilton_) _to face p._ 80 13. Crater of Vesuvius formed during the eruption of 1822 (_After Scrope_) 82 14. Crater of Vesuvius in 1756, from a drawing made on the spot. (_After Sir W. Hamilton_) 84 15. The summit of Vesuvius in 1767, from an original drawing. (_After Sir W, Hamilton_) 85 16. Summit of Vesuvius in 1843 86 17. Outlines of Vesuvius, showing its form at different periods of its history 87 18. Cascade of lava tumbling over a cliff in the Island of Bourbon. (_After Capt. S. P. Oliver, R.A._) 93