Salisbury. 299 
Drift of the North German Lowland. 
older than the upper, by some short period of time, at least. In 
support of this position, bowlder pavements and isolated bowlders 
in beds of till are cited, whose upper surfaces were striated by the 
passage of the ice-current over them, after they had ceased to 
move. The only reference which Dr. Wahnschatfe makes to the 
amount of material which can be carried beneath the ice at any 
given time is the citation of Penck’s opinion that till to the thick- 
ness of several meters may be carried beneath the ice at one time. 
We should have been glad of Dr. Wahnschaffe’s opinion concern- 
ing the positions in which ground moraine material is deposited, 
with reference to the margin of the ice. But to this point we find 
no reference. 
Two Glacial Epochs. The history of opinion concerning the unity 
or Otherwise of the glacial period is discussed, though less fully than 
geologists could have wished. The meagerness of the discussion at 
this point is probably the result of the fact that the object of the 
book is primarily geographic, rather than geologic. Dr. Wahn- 
schaffe believes that there were two glacial epochs, and most of 
the German glacialists hold the same view. The evidences cited 
for a bi-fold division of the drift, and therefore for two glacial 
epochs, are mainly (1) the existence of vertebrate remains in the 
beds of sand and gravel which separate beds of till in the vicinity 
of Berlin; (2) beds containing marine shells in like stratigraphic 
position at various localities in the northern part of Germany ; 
and (3) diatomaceous earths, likewise between beds of till. Most 
weight is attached to the first line of evidente (p. 87). Dr. Wahn- 
schaffe recognizes the possibility that the diatomaceous earths 
might have accumulated near the edge of the ice in situations tem- 
porarily abandoned by it, during the retreating phase of an oscil- 
latory movement. Under these circumstances, the next advancing 
phase of the ice’s oscillatory movement might bury the diatoma- 
ceous earths beneath a new bed of till. In view of this possibility 
Dr. Wahnschatfe points out the fact that these diatomaceous 
earths interbedded with till do not necessarily indicate an inter- 
glacial epoch. Neither does our author regard the deposits of 
marine shells as conclusive. Many of them are not known to be 
in the position in which they were deposited by the sea, and the 
various shell beds have not been correlated with each other with 
any degree of certainty. They appear to belong to more than 
one horizon. Those marine shell beds which retain the position 
