PREFACE TO THE FIFTH EDITION. 



The twenty years which have elapsed since the publica- 

 tion of the first edition have been exceedingly fruitful in 

 glacial investigations, as will be seen by consulting the bibliog- 

 raphy at the end of this volume. Nevertheless, as premised 

 in the preface to the first edition, these later investigations 

 have not seriously affected the main theories adopted twenty 

 years ago, but "pertain mainly to the details of the subject." 

 In the present revision the new material added is especially 

 abundant only upon a few subjects. Many existing glaciers 

 have been discovered in the Rocky Mountain system in the 

 United States and Canada — a region which w r as scarcely 

 touched by explorers until the close of the last century. 

 Explorations have also greatly extended our knowledge of 

 Alaskan glaciers while the changes in the Muir Glacier have 

 been so enormous as to be really startling, fully sustaining 

 the theoretical conclusions which I had drawn from my stud- 

 ies of the glacier in 1886. Much new material, also, has 

 accumulated concerning the glaciers of Greenland, Central 

 Asia, and the Antarctic Continent. 



As to the extent of the continental glaciers of the Pleisto- 

 cene period, there has been little additional information 

 since the publication of the first edition. Among the most 

 important additions has been the rectification of the glacial 

 boundary across New Jersey and Pennsylvania, where the 

 "fringe," or "attenuated border," imperfectly apprehended 

 by Lewis and Wright, has been carefully traced by Professor 



