324 Notices of Memoirs — W. 0. Crosby — 



unweatbered rocks. The prevailing colour of the ground-moraine 

 below the sharply defined limit of postglacial oxidation is that of 

 crushed rock, and not of residuary or other preglacial detritus. 



Probably no feature of the Greenland glaciers revealed to us by 

 Chamberlin's studies is of greater interest than the beautiful stratifi- 

 cation and lamination of the ice. This stratification is due chiefly to 

 the mode of distribution of the drift or rock debris, which forms 

 numerous relatively thin and continuous layers approximately 

 parallel with the bottom of the glacier, and often exhibiting flexures 

 and faults where the ground over which the ice moved was uneven. 

 This englacial drift is ground-moraine absorbed by the ice through 

 flexing and shearing movements. The debris is an element of weak- 

 ness, and tends to give rise to shearing- and gliding-planes. When 

 we consider how intimate this process of lamination-shearing is, 

 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 sufi"er faceting and striation 

 after the manner of the ground-moraine. Thus one supposed dis- 

 tinction between subglacial and englacial drift disappears. 



Comparison loith Modern Glaciers and Ice-sheets. 



That glaciers of the Alpine type are generally free from incor- 

 porated drift, other than that derived from lateral and medial 

 moraines through the agency of crevasses, is, no doubt, attributable 

 to the facts that their courses were long since swept relatively bare 

 of detritus, and that in their lower courses they are undergoing basal 

 melting, and hence depositing rather than absorbing drift. 



The numerous glaciers descending from the margin of the Green- 

 land ice-cap present two types: (1) the drift-laden glaciers which, 

 predominate north of latitude 76° ; and (2) the apparently drift-free 

 glaciers to the south. In the former, the drift is strictly a basal 

 feature, rarely rising to greater heights in the ice than 100 to 150 

 feet, even where the glacier may be a thousand feet or more in 

 thickness. Possibly the northern glaciers are more generally drift- 

 laden because the severe climate tends to prevent basal melting, and 

 a considerable part of it may represent the lower, di'ift-laden portion 

 of the ice-cap itself. The ice-cap virtually spills over the edge of 

 the plateau through deep V-shaped notches ; and the conclusion i8 

 unavoidable that a much larger proportion of the upper, clear, and 

 relatively mobile ice will flow down, than of the lower, drift-laden, 

 and relatively immobile ice. 



It is the general belief of geologists that if Greenland were divested 

 of its ice-cap it would exhibit continental relief — elevated margins 

 and a depressed interior. Probably very little of the englacial drift 

 rises to a greater height than 500 feet, or possibly 1,000 feet, even 

 when the thickness of the ice is one to two miles. None of the 

 suggested processes of absorption seem competent to difi"use the 

 detritus through a more considerable thickness of the ice. 



An important exception to this limitation of the range of englacial 

 drift should be made for the case when a later sedentary ice-sheet is 



