Davis.] 26 [May 3, 



these will as a rule correspond to the preglacial valleys. Undoubt- 

 edly their work has been effective in deepening old valleys, but it 

 can hardly aid in the formation of basins. Granting that a small 

 basin is begun, the standing water that must occupy all the pas- 

 sages in and below the ice will be inert of itself, and will act as a 

 cushion to streams falling from above, and prevent their cutting 

 to any great depth. Pot-holes or giant's cauldrons are the limit 

 of this kind of action. 



Charpentier showed that the fine mud of subglacial streams was an effect 

 of glacial action, by noting that these streams became nearly clear in late 

 winter when the motion and melting of the ice was decreased. (Essai sur 

 les Glaciers, 1841, 89.) 



Dollfus-Ausset (Materiaux pour l'etude des glaciers, 1864, I, 276) meas- 

 ured the fine sediment carried out by the stream from below the Unter-Aar 

 glacier, finding 132 grammes in a cubic meter of water. This corresponds to 

 a yearly rubbing off' of about 0.6 millimeter of rock from under all parts of 

 the glacier's basins ; or an erosion of one meter in 1666 years ; about two 

 and a half times as much as water could do in the same period. It is not 

 determined how much cf the sediment came from sand washed under the 

 ice by side streams, but it shows that the glacier does a considerable amount 

 of work. 



O. Heer describes the stream that runs out from the. Rosenlaui glacier 

 as having cut a channel deeper than the surface on which the ice rested. 

 (Die Urwelt der Schweiz, 1865, 582.) 



O. Fisher admits that glaciers may have cut out basins, but recognizes a 

 limit of depth where decrease of motion will balance increase of pressure, 

 and suggests that this limit will be the sooner reached on account of the 

 buoyant action of subglacial water. (The Formation of Lake Basins ; 

 Reader, Apr. 9, 1864 ; Apr. 25, 1865.) 



T. Belt suggested that the excavation of lake basins might be aided by 

 the falling of streams through the ice. (Geol. Soc. Journ., xx, 1864, 464.) 

 The idea is repeated by J. D. Kendall. (The Formation of Rock-basins, 

 Geol. Soc. Proc, 1879, 105.) 



Bonney has suggested that subglacial streams now carry more silt than 

 formerly, when surface fragments had less easy access to the bottom of the 

 ice. (Geol. Soc. Journ. xxvii, 1871, 322.) 



Ch. Grad describes stream channels on these rocky surfaces, showing 

 that the subglacial streams work faster than the ice. (Les Glaciers et l'ori- 

 gine des vallees. Club Alpin Franc. Ann., in, 1876, 474-479.) 



W. H. Niles limits ice erosion to small results, but allows subglacial 

 streams a considerable effect. (These Proceedings, xv, 1873, 378-381 ; xix, 

 1878, 335.) 



