19. Lava-stream (obsidian) in the Island of Vulcano, showing the imperfect liquidity of the mass 95 20. Interior of a rhyolitic lava-stream in the Island of Lipari, showing broad, sigmoidal folds, produced by the slow movements of the mass 96 21. Interior of a rhyolitic lava-stream in the Island of Lipari, showing the complicated crumplings and puckerings, produced by the slow movements of the mass 96 22. Vesuvian lava-stream of 1858, exhibiting the peculiar 'ropy' surfaces of slowly-moving currents. (_From a photograph_) _to face p._ 98 23. Vesuvian lava-stream of 1872, exhibiting the rough cindery surfaces characteristic of rapidly flowing currents. (_From a photograph_) _to face p._ 96 24. Concentric folds on mass of cooled lava. (_After Heaphy_) 100 25. Mass of cooled lava formed over a spiracle on the slopes of Hawaii. (_After Dana_) 100 26. Group of small cones thrown up on the Vesuvian lava-current of 1855. (_After Schmidt_) 101 27. Natural section of a lava-stream in the Island of Vulcano, showing the compact central portion and the scoriaceous upper and under surfaces 104 28. Section of a lava-stream exposed on the side of the river Ardèche, in the south-west of France. (_After Scrope_) 106 29. Portion of a basaltic column from the Giant's Causeway, exhibiting both the ball-and-socket and the tenon-and-mortise structure 107 30. Vein of green pitchstone at Chiaja di Luna, in the Island of Ponza, breaking up into regular columns and into spherical masses with a concentric series of joints. (_After Scrope_) 108 31. Illustration of the 'perlitic structure' in glassy rocks 109 32. Transverse section of a lava-stream 111 33. The Kammerbühl, or Kammerberg, Bohemia (as seen from the south-west) 113 34. Section of the Kammerbühl in Bohemia 114 35. Natural section of a volcanic cone in the Island of Vulcano 116 36. Section in the side of the Kammerbühl, Bohemia 118 37. Experimental illustration of the mode of formation of volcanic cones, composed of fragmental materials 120 38. Natural section of a tuff-cone, forming the Cape of Misenum, and exhibiting the peculiar internal arrangement, characteristic of volcanoes composed of fragmentary materials. (_After Scrope_) 121