DEDICATED TO DR. T. STERRY HUNT, F.R.S., Whose Work in the Chemical and Cosmical Relations of Geology is beyond all Praise, and is destined to command in the Future even greater Acceptance than in the Past. Various Theories as to Changes of Climate--The Astronomical Theory of Croll--The Geographical Theory of Lyell--Objections of a Geological Character to the Former--Testimony of Geology and Physical Geography in Favour of the Latter [Illustration: North America in Periods of Warm and Cold Submergence. (_A_) Early Cretaceous. (_B_) Glacial or Pleistocene. Shaded portion land. Unshaded portion.--Snow-clad mountains.--Crosses.--Ice-laden sea. These maps illustrate the probable geographical conditions of warm and cold periods. (p. 388.)] CHAPTER XIV. _CAUSES OF CLIMATAL CHANGE._ The subject of this chapter is one which has been in dispute ever since I began to read anything on geology, nearly sixty years ago. It ought to have been settled, but up to to-day one finds in geological works and papers--especially those relating to the Glacial age--the most divergent views; and in the writings of men not geologists, it is not unusual to find exploded theories gravely stated as established facts of science. The subject is one which I cannot hope to make interesting, but if the reader will wade through a short chapter, he will be able to find some of the data on which statements on this subject in other papers of this series are based. Mr. Searles V. Wood, in an able summary of the possible causes of the succession of cold and warm climates in the northern hemisphere, enumerates no fewer than seven theories which have met with more or less acceptance, and he might have added an eighth. These are:-- (1) The gradual cooling of the earth from a condition of original incandescence. (2) Changes in the obliquity of the ecliptic. (3) Changes in the position of the earth's axis of rotation. (4) The effect of the precession of the equinoxes, along with changes of the eccentricity of the earth's orbit. (5) Variations in the amount of heat given off by the sun. (6) Differences in the temperature of portions of space passed through by the earth. (7) Differences in the distribution of land and water in connection with the flow of oceanic currents. (8) Variations in the properties of the atmosphere with reference to its capacity for allowing the radiation of heat. Something may be said in favour of all these alleged causes; but as efficient in any important degree in producing the cold and warm climates of the Tertiary period, the greater number of them may be dismissed as incapable of effecting such results, or as altogether uncertain with reference to the fact of their own occurrence.