Davis.] 32 [May 8, 



Indiana and Illinois, in support of the Preglacial Origin of the Basins of 

 Lakes Erie and Ontario. Amer. Asscc. Proc, xxx, 1881, 23-35.) He 

 averages the estimates made independently in different counties by the 

 geologists of the several local surveys ; the result gives sixty-two feet for 

 the three States, or for Ohio alone, fifty-six feet. A considerable part of 

 this is local, and it is all, as will be shown later, within the region where the 

 ice acted more as a depositing than as an eroding agent : it moreover con- 

 tains the terminal moraines of the ice-sheet, and therefore gives an exag- 

 gerated measure of the thickness of detritus eroded from the country to the 

 north. The fine rock-flour, carried far down the Mississippi valley, and 

 not here included, must have been very considerable in total amount, but 

 cannot have been a large fraction of what remains. 



W. Upham estimates the mantle of drift that so generally conceals the 

 rock-outcrops in Minnesota as from one to two hundred feet thick ; it con- 

 tains a large part of pebbles from rocks near by. (Minn. Geol. Kept. 1879, 



5, 44.) 



G. H. Stone estimates the average thickness of the drift in Maine at 



thirty to fifty feet, and thinks that glacial erosion "reached only a few feet 

 below the limit of preglacial weathering." (Glacial Erosion in Maine, Port- 

 land Nat. Hist. Soc. Proc, 1881, 6, 11). 



Torell, Ramsay and Bauermann, a committee appointed by the British 

 Association to consider "Ice as an Agent of Geologic Change," reported a 

 method of gauging the work of existing glaciers, but expressed no opinion 

 as to past effects. Their suggestion requires long, laborious observation. 

 (Brit. Assoc. Rep. 1869, 1°, 171-174.) 



Dollfus-Ausset wrote " A la boue du glacier! a la terre vegetale dans la 

 civilization ! Sans glacier, pas de boue glaciaire, la roche a nu partout, et 

 si le glacier ne l'a pas deposee directement, c'est lui qui est l'auteur, le fab- 

 ricant de cette terre que nous cultivons. (Matdriaux pour 1' etude des gla- 

 ciers, V, 351 ; also, 417.) 



B. 3. Great Boulders. The large boulders of granite found on 

 the Jura mountains, and derived from the Central Alps, imply a 

 great transporting power in the glacier which carried, them, but 

 not necessarily an eroding power, as they probably rolled from a 

 mountain slope on to the ice below. But it is otherwise with the 

 enormous slabs of rock found in the drift of North Germany and 

 England, for these were displaced, broken, and bent by the 

 advance of the ice against them. The preglacial surface of 

 these regions was probably one in which valleys of a moderate 

 depth were sunk in generally horizontal rocks ; low abrupt cliffs 

 and outliers must have been common, as they are now in the Bad 



