GENERAL CONTENTS. 



CHAPTER XII. NATURE AND ORIGIN OF VOLCANIC ENERGY. 



Internal heat of the earth . 



Hypothesis of magmas . 



Action of radiated heat on the 

 earth's surface .... 



Mallet's theory of volcanic heat . 



Evidence in support of Mr. Mal- 

 let's views .... 



Temperature and pressure involved 

 in rock construction . 



PAGE 



1 68 

 169 



170 

 171 



172 

 173 



How upheaval facilitates the out- 

 burst of a volcano . . .175 



Linear arrangement of volcanoes . 176 

 Origin of the eruptive power of 



volcanoes 177 



Why eruptions are intermittent . 178 



Extinct volcanoes . . . 179 



CHAPTER XIII. MANIFESTATIONS OF VOLCANIC ACTION. 



Historic records : Graham Island 



arid Jorullo . . . . 180 

 Structure of a volcano and se- 

 quence of events . . .181 

 Steam and ashes . . . .182 

 Red clay in the deep sea . .183 

 Mud streams and bombs . .184 

 Lava streams . . . .185 

 Rate of flow and aspect of lava 



fields 186 



Cones and craters . . .187 



Volcanoes without cones 

 Fissure eruptions 

 Nearness of volcanoes to the sea . 

 Relation of volcanoes to springs, 



and decline of volcanic activity 

 Solfataras and mud springs 

 Petroleum springs 

 Geysers . . . 



Hot springs 



Relation of hot springs to mineral 



veins 



CHAPTER XIV. NATURE AND ORIGIN OF IGNEOUS KOCKS. 



Richthofen's hypothesis 



Hypothesis of igneous evolution . 



Views of Sir John Herschel and 

 Sterry Hunt .... 



Texture of volcanic rocks and tex- 

 tures of granite 



Comparison of granite and Clay . 



Evolution of igneous rocks . 



200 

 20 1 



202 



203 



205 



207 



Clarence King on American vol- 

 canic rocks .... 



Relation of plutonic and volcanic 

 rocks 



Igneous rocks related to sand- 

 stones 



Classification of igneous rocks 



Experimental formation of volca- 

 nic rocks 



1 88 

 189 

 190 



191 

 192 



193 

 194 



197 



198 



208 

 209 



211 

 212 



213 



CHAPTER XV. THE GRANITIC OR PLUTONIC GROUP OF ROCKS. 



Mineral composition of granite . 214 



Variation in chemical composition 215 



Varieties of granite . . 216 



Joints and decomposition . 217 

 Inclusions and concretions in gra 



nite 218 



Granite veins . . . 219 



Modes of occurrence of granite 220 



CHAPTER XVI. HISTORY OF BRITISH PLUTONIC ROCKS. 



Geological age of granite 

 Syenite .... 

 Mica syenite ; augite syenite ; else 



elite syenite 



Zircon syenite ; miascite 

 Diorite .... 

 Gabbro 



221 



222 



223 

 22 4 

 22| 

 226 



Age of igneous rocks . . . 

 Granite of Cornwall 



Elvans 



Groby syenite and Mount Sorel 



granite 



Granite of Anglesey . 

 Granites of the Lake District 



228 

 229 

 231 



233 

 234 

 235 



Minette and pikrite of the Lake 



District 237 



Granite in Scotland . . .238 



Arran . . . . , . 240 



Granites of Ireland . . . 242 



Granite veins .... 248 



Veinstones 250 



Phonolite of the Wolf Rock . 251 



