PREFACE TO VOLUMES II AND III. 



The salient features of the purpose and plan of this work were 

 set forth in the preface to Volume I, published two years ago. The 

 subject of that volume, Geological Processes and their Results, is 

 sufficiently distinct from the theme of these volumes, The History 

 of the Earth, to give occasion for additional prefatory remarks. Though 

 the subjects are thus measurably distinct, they have been given a 

 common bond by the treatment of the geological processes in a his- 

 torical way, on the one side, and by the development of the earth 

 history on dynamical lines, on the other. Not only has the history 

 been treated from a causal point of view in these volumes, but proc- 

 esses and principles have been discussed wherever the phase of the 

 history has seemed to make such discussion particularly pertinent. 

 Such special discussions have usually been introduced at the first stage 

 in the history at which the phenomena they are intended to eluci- 

 date were declared features, or at which their expression is well suited 

 to treatment. Sometimes, however, they have been delayed, to avoid 

 emphasizing too many dynamic subjects at a given stage. Some- 

 times, too, the dynamic treatment has been divided between suc- 

 cessive historical expressions, as when the subject is very complex, 

 or when different phases are best expressed at different historical stages. 

 This is the case, for example, with the dynamics of deformation, which 

 were treated at some length in Volume I, are further discussed in the 

 chapters on the origin and the early stages of the earth, are again 

 touched upon in connection with several periods when deformation 

 was pronounced, and are reverted to finally in the last chapter, in 

 their application to the peculiar phenomena of the continental borders. 

 A similar method is used in the treatment of climatic problems, par- 

 ticularly that of glaciation, which is considered at some length in con- 

 nection with the remarkable Permian phenomena, because of its advan- 

 tageous historical setting between antecedent and subsequent mild 



