958 HISTORICAL GEOLOGY. 



withiu 500 to 1500 feet of the bottom. It was intraglacial/ as now in Green- 

 land ; there was in general no superglacial drift over the ice-sheet. The local 

 exceptions to this occur over the melting lower margin ; for a short distance 

 about some "Nunatak" (page 240), where local melting had favored the 

 growth of alpine Algae ; and in regions reached by the dust of the drifting 

 winds. Even the stones and gravel, taken up from the bottom over which the 

 ice moved, might have been carried upward along oblique planes of bedding 

 or lamination into the ice-mass. 



A paragraph from the chapter on Glaciers (page 246) is here repeated because of its 

 apparent importance in connection with the accumulation, transportation, and deposition 

 of the drift. 



The slipping of the ice along planes of bedding or straticulation like that of the blue 

 bands has been shown by Forel to be a fact in several glaciers, among them the Bossons 

 Glacier at Chamouni. In the lower part of a glacier these planes have a dip upstream; 

 and as a consequence, the mass of the glacier, as it moves down the valley, rises by slip- 

 ping along one or more of the planes of lamellar structure. Forel observes that the fact 

 explains the difference of velocity between the upper and lo\ver beds of the ice ; the little 

 movement at the extremity of a glacier; the reappearance, at the surface, of bodies 

 buried in the interior of the glacier ; and the preservation of the thickness of the ice at 

 the lower extremity, notwithstanding the annual loss from melting. The cause must 

 have great influence over the direction of crevasses, and in all adjustments to resistances. 

 He states further that at the Glacier of Hochsbalm, a frontal moraine was formed in 1884, 

 by the slipping of a bed of clean ice over an old bed of debris-covered ice. (_Arch. Phys. 

 Nat. Geneve., 1889, xxii., 276, and Am. Jour. Sc, 1889, xxxviii., 412.) 



Besides taking up material for transportation, the glacier pushed along 

 bowlders and gravel wherever its mass rested, and especially where there 

 was a rocky surface at shallow depth below for it to slip over ; and the loose 

 material gathered, besides serving for abrasion, made a prominent part of the 

 ground-moraine here and there in progress of accumulation. 



The uneasy glacier stream — uneasy because forced to make unceasingly 

 new adjustments to the uneven surface underneath it — carried on the work 

 of corrasion among the transported stones with vastly greater force than 

 running water, because the ice had a firm hold on the stones and was plied by 

 pressure of vast amount. It was a wonderfully efficient rock-mill. The 

 stones, hard or soft, had their angles and surfaces rounded, and then were 

 gradually reduced to sand, earth, and rock-flour. Owing to this wearing out 

 of the stones, the drift in any region seldom contained stones gathered from 

 points more remote than the last fifty miles of travel. Shaler states that 

 the stones and bowlders on Xantucket were all gathered by the ice east of 

 Narragansett Bay. 



It is not surprising that, in Illinois, Indiana, and Iowa, where the distance 

 of travel from any good gathering-place was great, stones in the drift should 

 be few, and be almost confined to the hardest kinds, as those of chert ; that 

 the southern ice-limit should in some parts have no well-defined moraine ; 



1 The term englacial, used by some writers, is not here adopted because it is half Greek. 

 Intraglacial accords with Latin usage. 



