Why is not the formation of the sedimentary rocks capable of being observed? What is the first mode in which solid matter is taken up by water? Why are the waters of the ocean saline? What effect has the temperature of water in the solution of silex? What effect has an alkaline condition of water? [Illustration: Fig. 72.] What rock is soluble in water charged with carbonic acid? Give an instance of limestone formation from such solutions. How do rivers furnish sediment for the stratified rocks? What determines the position of rapids in rivers? What is the effect of waterfalls on the abrading action of rivers? What is the peculiarity of rock at Niagara which has prevented the fall from becoming a succession of rapids? What other circumstance increases the abrading action of rivers? What is the principal source of the sediment which is transported by rivers? What is the annual amount of sediment furnished by the Kennebec? The Merrimac? The Mississippi? The Ganges? What is the general tendency of these abrading forces? What is the effect of waves upon the coast, when it consists of unsolidified materials? Describe their effect upon rocky coasts. How is the encroachment upon such coasts shown? [Illustration: Fig. 73.] What is the effect of waves of less power? How are marine currents produced? How are they increased by the evaporation of the torrid zone? What are the most important marine currents? Which class of currents have the greater depth? Upon what does the power of deep currents depend? How would the effect of these currents be increased by earthquakes? Where will the effects of these currents be greatest? Mention instances of these effects. [Illustration: Fig. 74.] What must be the effect of such currents as the Gulf-stream and Mozambique channel? Mention, generally, the effects of these currents. Why does detrital matter remain suspended in the water of rivers? How is the coarse and fine sediment separated? Why do river currents extend some distance into the sea? What effect does this have in distributing the sediment which the rivers furnish? Upon what does the transporting power of marine currents depend? When a river enters a lake, why is its sediment deposited? Describe the effect. When is sediment deposited in the beds of rivers? Describe the effects of this deposition. Where is most of the sediment deposited? Give the area of some delta deposits. How do the deep-sea deposits now forming compare in extent with the earlier formations? State the several circumstances by which a succession of deposits would be arranged in strata. How are those differences produced upon which the separation into independent formations depends? Why are marine deposits nearly horizontal? [Illustration: Fig. 75.] How are the irregular stratifications produced? What peculiarity in the fossils will distinguish the lacustrine and marine deposits?