6 



or reject entirely the conclusions reached by their 

 fellows. 



The causes which have produced and are yet influ- 

 encing the climatic evolution of our planet are so funda- 

 mental and far-reaching in their consequences that a 

 failure to explain these causes and their mode of action 

 constitutes a serious defect in those branches of science 

 to which a study of these causes appertains. In the 

 present stage of Physical Geography and Geology, the 

 student is offered a mass of facts interpreted along vari- 

 ous lines, each interpretation disputed by high authori- 

 ties, and finally the results are summed up in the broad 

 conclusion that " we must be content to work and wait." 



SCOPE OF THE PKOBLEM. 



The problem of explaining this succession of climatic 

 variations is so attractive that there is perhaps none 

 other to which deeper thought has been directed, nor 

 upon which such diverse views are held. In its entirety 

 it constitutes one of the most far-reaching and grandest 

 problems of terrestrial physics. Nor is the scope of this 

 problem bounded by its relations to the earth. The 

 principles and laws involved in its solution must be gen- 

 eral. 



The development now reached by each one of the 

 planets can not be the same, and we find each in that 

 particular phase which its mass, environment, and ex- 

 posure have permitted it to reach. When we trace the 

 climatic history of the earth backward, the line of 

 research must lead into conditions now apparently exist- 

 ing upon planets in a less advanced stage. If we can 

 predicate the conditions towards which present climatic 

 developments are tending, these must be the apparent 

 conditions of a more advanced planet. Thus the stages 



