Valley of denudation, 87 Vein of segregation, 69 Verd-antique marble, 15 Volcanic rocks, 17, 25 of different ages, 26 in what states ejected, 25 Volcanic mountains, dimensions, 26 Volcanic activity-- regions of, 129 water essential to, 130 Volcanic cones-- formation of, 131 lateral, 132 Volcanic cinders, scoriæ, glass, 132 Volcanic action, effects of, 133 Volcanic origin-- of trappean rocks, 138 of granitic rocks, 138 Volcanoes-- number, 26 linear arrangement of, 131 near the sea, 129 new, 131 of the tertiary period, long active, 28 W. Watt on fusion of basalt, 100 Waves, action of, 107 Wealden, 49 Wind a geological agent, 96 Z. Zechstein, 47 QUESTIONS TO ELEMENTS OF GEOLOGY QUESTIONS. CHAPTER I. SECTION I. How many elementary substances are known? In what combinations is oxygen found? What proportion of the earth's crust consists of it? In what combinations does hydrogen occur? Nitrogen? Carbon? Sulphur? Chlorine? Fluorine? Iron? Manganese? How does silicium occur? Aluminium? Potassium? Sodium? Calcium? Magnesium? How are these elementary substances classified? (Silicium, or silicon, has but a doubtful claim to be regarded as metallic.) SECTION II. What is a simple mineral? How many are known? What are the physical properties of quartz? How are the several varieties distinguished? What are the physical properties of felspar? Mica? Hornblende? How are its varieties distinguished? Augite? Hypersthene? Talc? How are its varieties distinguished? Serpentine? Carbonate of lime? Gypsum? Its varieties? What other minerals are mentioned? SECTION III. Define the crust of the earth. Rocks. What are the unstratified rocks? What is the structure of granite? How are the varieties distinguished? What is the porphyritic structure? Describe hypersthene rock. What are volcanic rocks? Lava? Scoriæ? Pumice-stone? How is the vesicular structure produced?