108 



PROCEEDINGS OF THE PHILADELPHIA MEETING 



ure which falls within the estimates made by Coleman, Taylor, Lyell, Cham- 

 berlin, and Salisbury. It is a bit under those of Fairchild, Sardeson, and 

 Spencer and above those of Gilbert and Upham. It is considered a conserva- 

 tive figure. 



Estimated Duration of Pleistocene Oscillations 



Penck states that it must have been 16,000 to 24.()(X) years from the Buhl 

 stadium to the present, with 20,000 years as an average, and 25,000 to 40,000 

 years from the beginning of the Achen retreat to the present. In selecting a 

 figure, however, which shall be used as a unit of measurement in calculating 

 the duration of the entire Pleistocene period, he chooses 20,000 years as the 

 length of post-Wiirm time. 



The correlation of the mountain glaciations of the Alps with those of the 

 Scandinavian continental ice-fields of Pleistocene time has not been worked 

 out in all regions, but there is sufficient information at hand to saj- that there 

 were four advances of the continental ice over northern Europe which corre- 

 spond to tlie periods of ice-advance on the alpine forelands. Geikie remapped 

 in 1914 the llnd, Ilird, and IVth glaciation distribution in Europe. G. de 

 Geer delimited the retreating stages of the IVth glaciation in tlie Scandinavian 

 peninsula in 1912. » 



A correlation of American with European glacial deposits has been made by 

 Leverett. By considering with Leverett- the so-called lowan glaciation con- 

 temporaneous with the Illinoisan, it is possible to correlate the Giinz glaciation 



2 F. Leverett : Zeitsehrift fur Gletcherkunde, vol. iv, 1910, pp. 282-283. 



