68. What are _glaciers_? What thickness do they attain in the Alps? What is their rate of motion? What are _crevasses_, and how do they originate? What are _superficial moraines_? What are _terminal moraines_? What changes does a glacier effect upon its bed, and how are these modifications produced? What is the character of a glacial river? What is the origin of _icebergs_? How is the general absence of blocks and stones in Greenland icebergs to be explained? What is the nature of a submarine terminal moraine? What is the _ice-foot_? What is the chief agent in distributing erratic stones and blocks over the sea-bottom? What effect upon the sea-bed must stranding icebergs produce? 69. What are some of the chemical compounds held in solution in sea-water? Which of these go to form the shells and skeletons of marine animals? 70. Describe the action of breakers on a sea-coast. How does frost aid the wasting action of breakers? What effect has the nature of the rocks in the production of inequalities in a coast-line? Upon what part of the sea-bottom does the material derived by the action of the breakers chiefly accumulate? What effect have the tides and ocean currents in the distribution of sediment? 71. What is the general rule as regards fine-grained and coarse-grained deposits? Mention a partial exception to this rule. What effect have tidal currents in shallow seas? 72. How are rocks disintegrated through the action of plants? What is peat? What may be inferred from the occurrence of shell-marl underneath peat? What does the appearance of roots and trunks of trees, and of remains of land animals under peat, indicate? 73. What, generally, is the geological action of animal life? 74. What is coral? What is a _fringing_ reef? What is the general character of a _barrier_ reef? Give an example of one. What is an _atoll_? What is the nature of coral rock? What is Mr Darwin's theory of the formation of coral reefs? 75. What is the nature of the Atlantic ooze? In what respects may it eventually come to resemble chalk and limestone? Mention an instance of the abundant occurrence in the sea of animalcules with siliceous coverings and skeletons. What is the nature of the red clay found at great depths in the Atlantic and Southern Oceans? 76. What are some of the notions held in regard to the internal condition of the earth? At what (average) rate does the temperature of the earth's crust increase as we descend from the surface? 77. What is the nature of the movements to which the earth's crust is subjected? 78. Describe the hypotheses advanced to account for earthquakes. Mention some of the effects of earthquakes--1_st_, as regards alterations of level; and 2_d_, as regards modifications of the surface.