79. Mention a good example of tranquil elevation and depression of the earth's crust. Mention some of the proofs of an elevatory movement. Give proofs that shew depression of the land. How may certain former changes of sea-level be accounted for without inferring any movement of the land? 80. What effect must _depression_ have upon the strata forming the earth's crust? What is the result of a movement of elevation? What is the cause of _cleavage_? 81. What is the nature of the materials thrown out during volcanic eruptions? What is the general structure of a volcanic cone? How does molten rock make its escape from the orifice of eruption? What is the meaning of the terms _lapillo_, _puzzolana_, and _ceneri_? 82. What is lava? Describe the general appearance and mode of progression of a stream of lava. What effect is produced upon fragments of rock caught up and inclosed in lava; and what changes are caused in the pavement upon which it cools? How does a lava stream entering a lake or the sea behave in regard to the sediment gathering therein? To what is the basaltic structure due? How are the axes of the prisms in a columnar igneous rock arranged? Name some of the varieties of lava. What is the origin of the vesicular structure in igneous rocks? What portions of a bed of lava are most frequently scoriaceous? In what kinds of lava is the vesicular structure most abundantly met with? How have the vesicles become flattened? In what manner have they been filled with mineral matter? What is the origin of the dykes of modern volcanic districts? 83. How is metamorphism on the large scale supposed to have been induced? How may granite be at one and the same time a metamorphic and igneous rock? 84. Mention some of the views held with regard to the origin of mineral veins. 85. What is _denudation_? How do inclined strata prove that the strata have been denuded? How do _faults_ afford proof of denudation? What have been the general effects produced by denudation on the face of the land? 86. What part have the subterranean forces acted in the formation of mountains? To what geological action is the present aspect of these mountains due? What has determined the direction of river valleys? How have the valleys, dells, &c. been formed? What effect have faults had in determining the direction of river valleys? What is supposed to be the origin of the deep rock-basins occupied by many fresh-water lakes? How is the waste of land by denudation compensated? 87. What are _fossils_? What is meant by _petrifaction_? In what kind of rocks do fossils occur most abundantly, and in the best state of preservation? and what reason can be given for this? 88. How do fossils afford proof of varied physical conditions? Give a reason for some rocks being more barren of fossils than others. 89. State some of the characters which distinguish broadly the older fossiliferous strata from those similar accumulations which are being formed in our own day.