METHODS OF PALAEONTOLOGICAL INQUIRY. 63 



but concretely it is of the highest importance that they shall 

 be so named as to exhibit their relationship to the contempo- 

 rary formations of other continents. The problems concerning 

 the origin and migrations of genera, and the geographical dis- 

 tribution of mammals in general, can be solved only when the 

 chronological relations of geological horizons in different con- 

 tinents have been established. It is therefore necessary to use 

 classifications which shall not obscure these relations, and the 

 conservative terminology of some American writers has so far 

 misled European observers as to vitiate much otherwise excel- 

 lent work. For this reason I have preferred to follow the 

 French classification, even though it should introduce rather 

 startling innovations in our current systems. 



Time would utterly fail us to consider the whole of even the 

 Tertiary formations, and of these we must make a selection. 

 For our purpose none of the horizons is more suitable than the 

 White River. It is, in the first place, the classic ground which 

 yielded to Leidy and Owen the materials for their epoch-making 

 studies; it has been repeatedly explored for more than half a 

 century past, and is, therefore, the most thoroughly known of 

 all the formations, and it is much the richest in satisfactory and 

 well-preserved fossils. Finally, it happens to be the horizon 

 with which I am, personally, the most familiar, and therefore 

 speak of from a somewhat extended experience. 



The White River deposits offer many problems to the geolo- 

 gist which have not yet been solved, but into which it is not 

 necessary for us to enter. It will be convenient to consider 

 the body of water in which the beds were laid down as a lake 

 of fluctuating size, which at one time or other had a very great 

 extension. It covered northeastern Colorado, following the 

 foothills of the Rocky Mountains westward into central Wyo- 

 ming, sweeping thence along the southern edge of the Black 

 Hills of South Dakota, and eastward for a great, but as yet 

 unknown, distance into the plains, and covering very large areas 

 in Nebraska and South Dakota. Other areas of the same beds 

 in North Dakota may represent the same body of water, the 

 intervening strata having been swept away by denudation, but 

 this is still uncertain. 



