TACHYLYTE, 59 Tertiary volcanoes of British Islands, 276 _Terraces, sinter- and travertine-formation of_, 185, fig. 80 Temperature, increase in deeper parts of earth's crust, 335 -- rate of increase in different areas, 340 Teneriffe, 44, 151 _Teneriffe_, 178, fig. 77 -- _peak of_, 175, fig. 73 Tenon-and-mortise structure in basaltic columns, 107 Theodosius and Vulcano, 3 Thunder, accompanying volcanic outbursts, 28 Trachytes, 49, 50, 69 Trap-rocks, origin of, 241 Trass, 90 Travertine or Tibur-stone, 184 -- deposits of, 220 Triassic volcanoes of British Islands, 275 Tridymite deposited on lava-streams, 110 Troilite, 314 Troost, cited, 355 Tufa, or tuff, 90 Tuff-cones, character of, 157 -- denudation of, 157, 158 Typhon, fable of, 3 ULTRA-BASIC lavas, 50, 66 -- rocks, 317 VAL DEL BOVE, Etna, 133, 180, 209 -- -- _dykes in_, 134, fig. 55 Vapour-cloud over Vesuvius, 26, 29 -- -- Stromboli, 9 Ventotienne, Island of, section at, 130, fig. 49 Vesuvius, 37 -- changes in form of, 81 -- compared with Stromboli, 23 -- _crater of in 1756_, 84, fig. 14 -- -- _of in 1767_, 85, fig. 16 -- -- _of in 1822_, 82, fig. 13 -- -- _of in 1843_, 86, fig. 16 -- detonations at, 26 -- early history of, 83 -- eruption of year 79, 84 -- -- of 1822, 69 -- -- of April 1872, 24 -- -- of October 1822, 24 -- ejected blocks of, 45 -- first eruption of, 7 -- form of, 166 -- fossils of, 45 -- growth of cone of, 80 -- history of, 204 -- last eruption of, 7 -- lava-stream of 1855, 101 -- lava-streams of 1858, 1872, 97 -- lavas of, 104 -- minerals of, 46 -- -- ejected at, 149 -- observatory on, 24, 37 -- outlines of, 87 -- pine-tree appendage of, 29 -- scoria-cones in lava, 122 -- -- on lava of 1855, 153 -- steam emitted by lava of, 27 -- vapour-cloud over, 26, 29 Vesuvian stage, 23 -- _eruption, photograph of_, 24, fig. 5 Viscid lavas of Lipari Islands, 94-96 Vitreous lavas, devitrification of, 259 Volcanic action, laws of, 32 -- bombs, 70, 71 -- cycles, nature of, 221, 222 -- -- duration of, 223 -- cones, internal structure of, 115-122 -- -- _experimental illustration of formation of_, 120, fig. 37 -- -- limits to height of, 166 -- -- form of, 152 -- -- dimensions of, 152 -- -- irregular development of, 90 -- -- slopes of sides of, 91 -- -- composed of ejected rock-fragments, 156 -- -- curved slopes of, 156 -- _débris_ on sea-bottom, 240 -- dust, fineness of, 69 -- districts, areas of upheaval, 245 -- ejections, alteration of, 258 -- eruptions, compared to ebullition, 19, 20 -- forces, compensate for denudation, 283 -- -- intensity at former periods, 278 -- -- necessity for action of, 285 -- -- shifting of from one area to another, 277 -- mountains, origin of conical forms, 89 -- -- mode of growth, 89 -- phenomena of the past similar to those at present, 273