CHAPTER XII.. The Attempts Which Have Been Made to Explain

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

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

39. Section of a small scoria-cone formed within the crater of Vesuvius in the year 1835, illustrating the filling up of the central vent of the cone by subsequent ejections. (After Abich) 122

40. Volcanic cones composed of scoriæ, and breached on one side by the outflow of lava-currents. (After Scrope) 128

41. Campo Bianco, in the Island of Lipari. A pumice-cone breached by the outflow of an obsidian lava-stream to face p. 124

42. Volcanic cones in Auvergne, which have suffered to some extent from atmospheric denudation. (After Scrope) 124

43. Experimental illustration of the mode of formation of volcanic cones composed of viscid lavas. (After Reyer) 126

44. The Grand Puy of Sarcoui, composed of trachyte, rising between two breached scoria-cones (Auvergne). (After Scrope) 126

45. Volcanic cone (Mamelon) composed of very viscid lava (Island of Bourbon). (After Bory de St. Vincent) 127

46. Another Mamelon in the Island of Bourbon, with a crater at its summit. (After Bory de St. Vincent) 127

47. Cliff-section in the Island of Madeira, showing how a composite volcano is built up of lava-streams, beds of scoriæ, and dykes. (After Lyell) 125

48. Section seen at the cascade, Bains du Mont Dore. (After Scrope) 130

49. Section in the Island of Ventotienne, showing a great stream of andesitic lava overlying stratified tuffs. (After Scrope) 130

50. Cliff on the south side of the Island of San Stephano 131

51. The headland of Monte della Guardia, in the Island of Ponza 131

52. Western side of the same headland, as seen from the north side of Luna Bay 132

53. Sea-cliff at Il Capo, the north-east point of Salina, showing stratified agglomerates traversed by numerous dykes, the whole being unconformably overlaid by stratified, aqueous deposits 137

54. Section observed in the Val del Bove, Etna, showing a basaltic dyke, from the upper part of which a lava-current has flowed 138

55. Basaltic dykes projecting from masses of stratified scoriæ in the sides of the Val del Bove, Etna 134

56. Sheets of igneous rock (basalt) intruded between beds of sandstone, clay, and limestone (Island of Skye) 137

57. Plan of the dissected volcano of Mull in the Inner Hebrides to face p. 142

58. Section of the volcano of Mull along the line A B " 142

59. Summit of the volcano of Monte Sant' Angelo, in Lipari, exhibiting a crater with walls worn down by denudation 158

60. Outlines of lava-cones 160

61. Diagram illustrating the formation of parasitic cones along lines of fissure formed on the flanks of a great volcanic mountain 162

62. Outline of Etna, as seen from Catania 162

63. Outline of Etna, as seen from the Val del Bronte 163

64. Plan of the volcano forming the Island of Ischia 163

65. A primary parasitic cone, with a secondary one at its base--Ischia 164

66. Scoria-cone near Auckland, New Zealand, with a lava-current flowing from it. (After Heaphy) 165

67. Section of rocks below the ancient triassic volcano of Predazzo in the Tyrol 165

68. Cotopaxi, as seen from a distance of ninety miles. (After Humboldt) 168

69. Citlaltepetl, or the Pic d'Orizaba, in Mexico, as seen from the Forest of Xalapa. (After Humboldt) 169

70. Lac Paven, in the Auvergne. (After Scrope) 171

71. The crater-lake called Lago del Bagno, in Ischia, converted into a harbour 172

72. Lake of Gustavila, in Mexico. (After Humboldt) 172

73. Peak of Teneriffe, surrounded by great crater-rings. (After Piazzi-Smyth) 175

74. The volcano of Bourbon, rising in the midst of a crater-ring four miles in diameter. (After Bory de St. Vincent) 176

75. The volcano of Bourbon, as seen from another point of view, with three concentric crater-rings encircling its base. (After Bory de St. Vincent) 176

76. Vesuvius as seen from Sorrento, half encircled by the crater-ring of Somma 177

77. Outlines of various volcanoes illustrating the different relations of the craters to cones to face p. 178

78. Island thrown up In the Mediterranean Sea in July and August, 1831. (After the Prince de Joinville) 179

79. Sinter-cones surrounding the orifices of geysers 183

80. Diagram illustrating the mode of formation of travertine- and sinter-terraces on the sides of a hill of tuff 185

81. Map of the volcanic group of the Lipari Islands, illustrating the position of the lines of fissure upon which the volcanoes have been built up 192

82. The Puy de Pariou, in the Auvergne, illustrating the shifting of eruption along a line of fissures 193

83. Ideal section of the Puy de Pariou 194

84. Fissure formed on the flanks of Etna during the emotion of 1865. (After Silvestri) 194

85. Plan of the Island of Vulcano, based on the map of the Italian Government 196

86. Vulcanello, with its three craters 197

87. Section of basalt from Ovifak, Greenland, with particles of metallic iron diffused through its mass 319

88. Diagram illustrating the relations between the terrestrial and the extra-terrestrial rocks to face p. 322

89. A group of sun-spots. (After Secchi) 362

90. A sun-spot, showing the great masses of incandescent vapour rising or falling within it. (After Secchi) 363

91. The edge of a sun-spot, showing a portion of the prominent masses of incandescent gas (A) which detached itself at B and floated into the midst of the cavity. (After Norman Lockyer) 363

92. Drawing of a solar prominence made by Mr. Norman Lockyer, March 14, 1869, at 11 h. 5 m. A.M. 364

93. The same object, as seen at 11 h. 15 m. on the same day. (After Norman Lockyer) 365

94. Drawings of a solar prominence at four different periods on September 7, 1871. (After Young) 366

95. A group of Lunar craters (Maurolycus, Barocius, &c.), the largest being more than sixty miles in diameter 368

VOLCANOES.