296 The American Geologist. May, 1802 
tions below, reveals the interesting fact that in many places the 
upper surface of the Prepleistocene formations is below the level 
of the sea. Were it not for the drift, a very considerable portion 
of porth Germany would be covered by salt water. In several 
places where the depth of the drift is known, its lower surface is 
more than 100 meters below the level of the sea. In one place 
its lower surface reaches the astonishing depth of 169 meters be- 
low sea-level. 
The disturbances of the Prepleistocene formations by ice-pres- 
sure during the glacial period are discussed. ‘The conclusion is 
reached that such disturbances, resulting in the folding and dis- 
torting of strata, were widespread, but that they did not attain 
such proportions as to exert an important influence upon the geog- 
raphy of the region atfected. 
Thickness of drift. The chief interest in the book centers in 
the discussion of the glacial formations themselves. The thiek- 
ness of the drift) presents variations not less than those which 
characterize the corresponding formation in our own country. [In 
many localities the depth of the drift is more than 100 meters. 
The greatest depth given is 171 meters, but even at this depth the 
bottom of the drift had not been reached at this locality. One 
hundred and seventy-one meters is somewhat more than the great- 
est thickness of the drift known to the writer in the United States. 
The average depth of drift on the north German ‘‘flatland”’ is 
probably greater than that over most areas of equal extent in our 
own country. This is no doubt in part. the result of the softer 
character of the formations over which the ice there spread. 
They were more easily eroded, and to a greater depth were con- 
verted into drift. 
At the beginning of the chapter which discusses the effect of 
the ice upon the geography of north Germany, the author reviews 
the history of opinion concerning the drift of Germany. This 
involves a review of the history of opinion concerning the glacial 
hypothesis, from the time when it was shown to be impossible, to 
the time when, in spite of its impossibility, it was demonstrated. 
This is not the first time that that which was believed to be matli- 
ematically impossible has proved to be true, particularly in geol- 
ogy. Concerning the application of mathematics to geology Dr. 
Wahnschatfe quotes approvingly the statement of F. von Rich- 
thofen: ‘‘Mathematical caleulation is inclined to take too little 
