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. CHAPTER I. INTRODUCTORY: NATURE OF THE INQUIRY. 'What is a volcano?' This is a familiar question, often addressed to us in our youth, which 'Catechisms of Universal Knowledge,' and similar school manuals, have taught us to reply to in some such terms as the following: 'A volcano is a burning mountain, from the summit of which issue smoke and flames.' Such a statement as this, it is probable, does not unfairly represent the ideas which are, even at the present day, popularly entertained upon the subject.