TERTIARY MAMMAL HORIZONS. 35 



Upper Pleistocene : Post Glacial. 



Middle Pleistocene : Glacial. 



Lower Pleistocene : Preglacial. 



Briefly, the glacial stoiy presented in the second column of 

 our Table is as follows: (i) The preglacial stage presents a 

 mingling of south temperate, temperate and northern forms. 

 (2) The long first glacial advance was followed (Pohlig) by the 

 Rixdorf stage, intermorainal, colder than the succeeding Mos- 

 bach and Thuringian stages which have a more temperate facies 

 in the recurrence of some of the Forest Bed fauna. (3) The 

 faunal evidence for a colder mid-glacial period is conclusive ; the 

 evidence for a second or mid-glacial advance, between the first 

 and last great glacial stages, is mainly biological, that is sub- 

 arctic are followed by more temperate life forms, as we gather 

 largely from studies of the rodent fauna by Nehring, Studer 

 and others. The hypothesis of three distinct glacial advances 

 and of two interglacial retreats rests therefore upon a combina- 

 tion of geological and biological evidence which is not as yet 

 conclusive. We shall consider it more fully after discussing the 

 fauna. It is supported geologically by observations of Penck 

 and Bohm in the Bavarian Alps. Upon this theory the Pleisto- 

 cene history with its fluctuations of temperature is epitomized in 

 the following Table. This Table is an attempt to combine the 

 chief results of the masterly work of Dawkins, Pohlig, Boule, 

 Nehring, Studer, Woldrich, Schlosser, and others. None 

 of these authors has treated the whole period ; yet there is an 

 evident harmony and synthetic trend in their -w^ork. 



Deposits. — Geologically we have to do with the characteristic 

 glacial deposits, boulders, boulder clay or drift, gravels and till. 

 The origin of the fine calcareous loam termed " Loess" distin- 

 guishing the upper middle Pleistocene is still under debate ; it is 

 partly glacial mud ; partly subaerial, it is also subsequent to the 

 second glacial stage, and in part postglacial. We find also the 

 river deposits of the lower and mid-Pleistocene (Forest Bed, 

 and Mosbach) as well as of all higher divisions. The mid-Pleis- 

 tocene was distinguished by volcanic disturbance, as attested in 

 Thuringia by the volcanic travertines and tufas. There are also 



