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

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.