How is it known that there has been no period of universal disturbance? SECTION III. How is it known that the mountains have been covered by the ocean? [Illustration: Fig. 57.] [Illustration: Fig. 58.] Were the granitic ridges thus covered? Has the level of the sea been, to any considerable extent, fluctuating? How, then, have the rocks, of which the mountain masses consist, been covered by sea? Give the evidence that different mountains were elevated at different times. Has the process of upheaval been sudden or gradual? How are the mountain valleys, which have the direction of the mountain ranges, been produced? [Illustration: Fig. 59.] How is the existence of submarine mountains shown? What is the movement by which continents are elevated? State the evidence of the elevation of continents from the existence of elevated sea-beaches. The evidence of the elevation of the coast of Maine. The evidence of elevation from the existence of lakes. From the geographical range of the older strata. The evidence of the recent elevation of South America. Of the rising of the north of Europe. State the proof of subsidence from the occurrence of submerged forests. Why are these changes but little observed? [Illustration: Fig. 60.] What are the grounds for asserting that a change of level is taking place over a large area in the Pacific and Indian Oceans? [Illustration: Fig. 61.] [Illustration: Fig. 62.] [Illustration: Fig. 63.] [Illustration: Fig. 64.] What is the present state of the coast of Greenland in this respect? Have the changes of level of the same place always been in the same direction? Give the evidence of elevation and depression in South America. In Italy. [Illustration: Fig. 65.] What general conclusion may we draw in respect to the stability of the earth's surface? To what extent can we ascertain the geography of past epochs? What former relations of land and water are suggested as not improbable? SECTION IV. How can we estimate the denudation which the igneous rocks have suffered? How do faults indicate the denudation of the stratified rocks? How do valleys indicate denudations? Describe the instance in Scotland. What is the evidence of denudation in the Connecticut valley? How are valleys produced? [Illustration: Fig. 66.] What is the condition of the surface rock in the colder portions of the temperate zones? [Illustration: Fig. 67.] [Illustration: Fig. 68.] With what is the surface rock generally covered? How are soils formed? How may soils be improved? What is necessary to render soils fertile? SECTION V. What means have we of judging of the climate of former periods? What was the climate of the coal period? What animal fossils indicate a former warm climate? What evidence that Siberia once enjoyed a milder climate? Do similar indications appear in the southern hemisphere? When has the climate of the earth been most uniform?