Has the climate been growing gradually colder to the present time? What is the evidence of a somewhat recent period of intense cold? What recent local changes of climate are mentioned as having occurred? SECTION VI. State the general advantages of geological changes. By what changes have the coal-beds and other stratified rocks become accessible? What advantage from these elevating forces in reference to the granitic rocks? [Illustration: Fig. 69.] How do these changes affect our means of knowing the structure of the earth? Explain the origin of springs, wells, and artesian wells? By what changes have the metallic ores become accessible? In what light, then, are we to regard disturbances of geological structure? CHAPTER IV. What is the object of the preceding chapters? How can we arrive at a knowledge of the causes which have produced geological phenomena? Have geological causes always operated with the same intensity? How are the means of forming correct geological theories increasing? SECTION I. How does oxygen become an agent in the disintegration of rocks? How does carbonic acid operate in the disintegration of rocks? What is the effect of moisture and rain? What is the effect of variations of temperature? What other atmospheric causes are mentioned? How do these causes become important? What are some of their effects? SECTION II. What are the changes which are to be referred to chemical agency? Mention some of the disturbances which give rise to chemical changes. What are the principal effects of chemical action? How is the cleavage structure accounted for? [Illustration: Fig. 70.] [Illustration: Fig. 71.] Mention instances which show that a cleavage may be established in a body in a solid state. In it a crystalline arrangement of the particles of the mass? What other divisional planes exist in rocks? Mention instances of concretionary formations. Why may not these concretions have been deposited as nodules? How have these concretions been formed? Mention instances of segregation without the concretionary structure. How was the segregation in these instances effected? How is the columnar structure produced? What is the origin of the mineral veins which are first mentioned? How is it shown that other veins are not injected? How were these veins formed? What is the force by which these molecular changes have been effected? SECTION III. In what ways are geological changes produced by human agency? Of what are the organic remains, in rocks, the record? What rocks contain organic materials in large quantity? What is the most abundant organic product? Explain the mode of growth of corals. Give instances of extensive coral reefs. What is the total amount of surface covered by the coral reefs? SECTION IV. What degree of importance is attached to water as a geological agent? What are the sources of the sediment which water deposits?