304 The American Geologist. May, 1892 
beds of till separated by layers of sand may arise within the 
period of one glaciation, we shall be likely to dissent from his 
implied (though not explicitly stated) conclusion, that every bed 
of sand interstratified with till, records a retreat of the ice, bar- 
ing the surface on which the sand accumulated, and that each bed 
of till overlying such bed of sand, records a re-advance of the 
ice. IT am not authorized to speak for American glacialists in 
general, but IT hold it altogether possible that the deposition of 
till may be succeeded by the deposition of stratified sand, and 
this again by till, beneath the marginal portion of the ice, inde- 
pendent of any change in the position of the ice’sedge. If each 
of the several layers of till which may locally alternate with sand 
were continuous over wide areas, and if the intervening layers of 
sand were also continuous over wide areas, oscillations of the ice 
margin would seem to best explain the phenomena. Dr. Wahn- 
schaffe does not indicate whether or not this is the condition of 
things in Germany. It would be a condition of things most diffi- 
cult of demonstration if true. From my acquaintance with the 
German drift, | do not think it generally true, and I see no reason 
for assuming an oscillation of the ice’s edge for each change from 
ice to water deposition, within the body of the last glacial drift. 
It is not to be understood that the writer is arguing against osceil- 
lations of the ice’s edge. Such oscillations, both seasonal and 
periodic, are believed to have occurred, and these oscillations may 
have given rise to many alternations of till and sand. But it is 
not deemed necessary to assume oscillations of the ice’s edge to 
explain all alternations of fill and sand. 
Topography of the ground moraine. Two distinct types of 
topography are represented by the ground moraine, according to 
the volume before us. In the one case the surface is plain or 
but shghtly undulatory, and more or less dissected by valleys, 
some of which are dry. Within those areas of ground moraine 
where the topography is of the plainer type, there are occasional 
sharp sinks of limited size, sometimes occuring singly, and some- 
times in series. Many of these depressions have become the seat 
of ponds or bogs. Their existence is attributed to the action of 
water plunging down through crevasses from the surface of the 
ice, and wearing hollows in the land surface below. 
The second type of topography which characterizes the ground 
moraine, as classified by the German geologists, is designated 
