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



the present time. The advocates of unity should present 

 direct evidences of indivisibility and of the persistence of like 

 conditions throughout the period if they are to merit attention. 

 Destructive arguments have their place in such discussions, 

 but they have an inferior value, unless they are attendants of 

 affirmative arguments. 



Prof. "Wright's introductory statement of the question seems 

 to me to need radical reconstruction to be even approximately 

 representative of the present attitude of glacialists. If we set 

 aside the views of those who hold glacio-natant theories of the 

 origin of the drift, in whole or in part, there will remain at 

 least four classes of views, with an ulterior fifth class. These 

 may be designated, (1) the primitive views of unity, (2) the 

 later views of unity, (3) the several views of duality. (4) the 

 several views of plurality or diversity, and (5) ulterior syn- 

 thetic views based on exhaustive analysis. 



1. Primitive -views of unity. — The old views of unity 

 recognize but a single comparatively short ice invasion (modi- 

 fied by oscillations of the margin), involving but one stage of 

 land elevation followed by one stage of depression, the Cham- 

 plain. Local glaciation was held to be an incident of the 

 retreat, and an elevation was thought to accompany the suc- 

 ceeding Terrace epoch. The stages of elevation and depres- 

 sion were quite generally held to have a genetic relationship 

 to glaciation, but that was not universal, nor is it essential to 

 the classification. It was the view of some that local glacia- 

 tion accompanied the elevation of the Terrace epoch, thus 

 constituting a species of second glaciation, but this is imma- 

 terial so far as the interpretation of the great body of drift in 

 the United States is concerned 



2. Later views of unity. — The later views of unity depart 

 somewhat radical!} 7 from the old ones in postulating a long 

 depression, or series of depressions, in the earlier stages of 

 glaciation (perhaps preceded by a stage of elevation), followed 

 by a prolonged stage or series of stages of elevation followed 

 again by a stage of depression, this last being the Champlain 

 depression in the strict sense of the term. This view differs 

 from the old doctrine of unity in the important and very 

 necessary feature of recognizing at least one early stage of 

 depression, of which several separate episodes of glaciation 

 have already been determined. It also parts company with 

 the old view in entertaining a radically different conception of 

 the extent and complexity of the period and of the import- 

 ance of its constituent episodes. The working methods of 

 those who hold this view are radically different from those of 

 the old school, as well as their habits of interpretation, in that 

 they are analytical and discriminative in respect to structural, 



