T. C. Chamberlin — Diversity of the Glacial Period. 173 



genetic and historical factors. In these respects, this view 

 does not very essentially differ from the following views. It 

 only diverges from them in recognizing the whole complex 

 series of glacial movements as connected and bound together 

 across the episodes of retreat and advance by conditions that 

 are to be interpreted as signifying continuity rather than dis- 

 continuity. 



3. Duality. — The several views of duality differ in detail, 

 but they agree in recognizing one, and only one, epoch of 

 deglaciation of such nature and importance as to justify a 

 division of the period into epochs by means of it. Each of 

 the two epochs is held to have been marked by episodes of ice 

 advance and retreat and other changes, but these are thought 

 to be subordinate to a bipartite division. The separation of 

 the epochs of lake-formation in the Great Basin region by an 

 interval of aridity, the separation of the epochs of glaciation 

 of the interior by an interval of extensive and high gradient 

 erosion and weathering, and the intervention of a temperate 

 fauna and flora between glaciated deposits in Europe, are 

 typical illustrations of the class of data to which these views 

 appeal. It is not essential to prove or to hold that the ice 

 completely left the continent, any more than it is necessary to 

 prove or to hold that the sea retreated from the land between 

 the recognized epochs of the Niagara period. The question is 

 not a question of two glacial periods but of two glacial epochs, 

 as these terms have come to be commonly used in America 

 and England. This does not make the distinction merely a 

 choice of terms. It is a distinction of ideas, and of not unim- 

 portant ideas. The dualist holds that he has grounds for an 

 important subdivision of glacial history comparable in signifi- 

 cance to that which divides the periods preceding. There is, 

 it is true, an element of judgment here, as in all similar cases, 

 but back of that there is an essential distinctness of ideas. 



4. Plurality or diversity. — Some of the most experienced 

 glacialists of Europe and of this country who have held to 

 the last class of views, have come to feel their inadequacy in 

 the interpretation of the vast growing mass of evidence, and 

 have enlarged their views in the direction of still greater 

 diversity. These have come either to recognize three or more 

 glacial epochs, or to feel that the diversity is so great as to 

 make a simple bi-partite division unsatisfactory. One of the 

 latest and most notable expressions of this class of views is to 

 be found in a recent paper by Dr. James Geikie, " On the 

 Glacial Succession in Europe'' (Trans. Roy. Soc. Edinburgh, 

 May 16th, 1892) in which five glacial epochs are recognized, 

 and a map of the distribution of the ice in the second and 

 fourth of these epochs is given. 



