Engl tidal Drift. — Crosby. 
208 
Caverns are indeed common in all formations, and cases anal¬ 
ogous to that indicated by the Postville well can be easily 
recalled by every observer. The occurrence at Davenport of 
Carboniferous shales between beds of Devonian limestone, as 
described by Hall and Barris, and the similar shales, described 
by Farnsworth, at Clinton in caverns excavated in the Niag¬ 
ara dolomite, demonstrate the possibility of numerous appa¬ 
rent anomalies of stratification in well records, and at the 
same time suggest, for some of them at least, a very natural 
and simple explanation. 
ENGLACIAL DRIFT. 
By W. O. Crosby. Boston, Mass. 
Introduction. 
Among the unsolved problems of glacial geology none, per¬ 
haps, are more important or pressing at the present time than 
the relative abundance and significance of the englacial drift. 
Although this subject has received its due share of attention 
in the recent literature of the science, the views of the lead¬ 
ing glacialists are still strongly contrasted. Thus Chamberlin 
and others hold that the englacial drift was exceedingly 
scanty in amount, consisting chiefly of a few far-travelled, 
angular and unglaciated bowlders now scattered over the sur¬ 
face of the drift; while Upham, the foremost exponent of the 
opposing theory, finds in the englacial drift the chief source 
of all the manifold forms of modified drift and also of drum- 
lins. 
The arguments of those who minimize the englacial drift 
are based chiefly upon the local character of the drift, the 
supposed paucity of englacial drift in modern glaciers, and 
the mechanical difficulty of accounting for a differential up¬ 
ward movement in the ice-sheet whereby large volumes of 
basal drift or ground moraine became englacial. The cogency 
of these arguments is beyond question; and a careful study 
of the recent literature satisfies me that, as the case now stands, 
the onus probandi may fairly be said to rest upon those who 
regard the englacial drift as an important factor in Pleis¬ 
tocene geology. Still, the englacial drift accounts so satis¬ 
factorily for far-travelled erratics, and the derivation from it 
