CHAPTER XIV. 

 CAUSES OF CLIMATAL CHANGE. 



THE subject of this chapter is one which has been in dis- 

 pute ever since .1 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. 



