256 THE ICE AGE IN NORTH AMERICA. 



showed that the quantity of mud varied in the different streams, 

 and even in the same river. The result of ten different obser- 

 vations in the months of June and July gave a mean of 147*9 

 grammes of mud in 1 cubic metre of water. (See "Quarterly 

 Journal of the Geological Society," 1877, p. 157.) * 



Mr. J. E. Marr gives the following facts concerning the 

 extent to which erosion is proceeding beneath some of the 

 Greenland glaciers : 



The erosive power of an ice-sheet is well seen by a glance 

 at the observations made upon the rivers which flow into the 

 fiords of Nagsugtok and Isortok. and which have their origin 

 at the ends of the tongues of ice which occupy the val- 

 leys continuous with these fiords. The river from the first 

 contained only 200 to 225 grammes of mud per cubic metre of 

 water in the month of July ; while the second, in the month 

 of June, inclosed 9,129 to 9,744 grammes. This is compared 

 with the amount carried by the Aar where it emerges from 

 the glacier : it there contains only 142 grammes. The great 

 flifference presented by the rivers which fall into the two fiords 

 is attributed to the fact that the ice moves with much greater 

 speed toward the fiord of Isortok than toward that of Nagsug- 

 tok. It is calculated that the quantity of fine mud carried 

 into the former of these fiords amounts to 4,062,000,000 kilo- 

 grammes per day. This mud is deposited in the interior of 

 the fiord, which is filled up to such an extent in its upper por- 

 tion that even flat-boats can not pass up it. f 



The amount of material carried to the sea by the subgla- 

 cial streams during the continuance of the Glacial period 

 in North America could not be estimated, even though we 

 knew the rate of transportation, unless we had more definite 

 ideas than we now have of the length of time during which 

 glacial conditions prevailed. But if, as is probably the case, 

 the deposits of loess in the valley of the Mississippi are of 



* Geikie's " Prehistoric Europe," pp. 231, 232. 



f See "Geological Magazine," April, 1887, summarized in " American Jour- 

 nal of Science," vol. cxxxiv, 1887, p. 313. 



