Englacicil Drift. — Crosby. 
223 
producing in extreme cases as many as twenty distinct layers 
in an inch, it can hardly be doubted that the englacial rock 
fragments, more especially if of small size, must suffer facet¬ 
ting and striation after the manner of the ground moraine. 
Thus one supposed distinction between subglacial and engla¬ 
cial drift in a measure disappears and is no longer available 
as an argument to minimize the englacial drift of the Pleisto¬ 
cene ice-sheet. 
Professor Chamberlin has noted* that while the Greenland 
glaciers commonly slide over the ground moraine in their 
lower courses, allowing it to accumulate beneath them, they 
appear in their upper courses to drag and carry it along, fit¬ 
ting snugly in their respective valleys and scoring the ledges 
over which they move. By parity of reasoning we may sup¬ 
pose that, although at its lower extremity the ice is observed 
to slide over the interstratified debris, farther back from the 
margin the debris, being frozen in the ice, is urged along and 
glaciated by the motion of the ice. 
I have observed something analogous to this lamination 
shearing of glacial ice in the obsidian flows of the Lipari is¬ 
lands. In consequence, perhaps, of incipient crystallization, 
or of partial relief from pressure, due to the fact that the lava 
continued to flow after it had begun to stiffen, imprisoned or 
dissolved aqueous vapor is liberated along certain planes co¬ 
incident with the plane of flowing, giving rise to layers of 
vesicles. These vesicular layers (analogous to debris layers 
in the glacier) become, with continued stiffening of the 
magma, relatively planes of weakness and hence shear planes, 
—a true flowing movement which alfects the entire bqdy of 
magma being transformed in part, by continued pressure from 
behind, to mechanical shearing along definite planes. 
Comparison with Modern Glaciers and Ice-sheets. 
Perhaps the most cogent argument against the view that the 
drift of the Pleistocene ice-sheet was, during certain pro¬ 
longed phases of its history, largely or chiefly englacial, is 
that based upon the comparative rarity of englacial drift in 
modern glaciers, and, seemingly, in modern ice-sheets. The 
general freedom of glaciers of the alpine type from incorpo- 
* Journal of Geology, hi, 67, and 208-210. 
