DJSUJILIXS. 293 



now found. The compactness of the mass suggests an origin 

 under heavy pressure ; the attitude of the hills demonstrates 

 a close dependence on the motion of the ice-sheet : the super- 

 position of kames on their flanks proves that their present 

 form was essentially completed when they were uncovered by 

 the ice-sheet, and the small change of form in the kames shows 

 that the drumlins also can have suffered very little from post- 

 glacial erosion ; the faint channeling of their smooth slopes by 



rain measures the small amount of denudation that thev have 



j 



suffered since they were made. It must therefore be concluded 

 that they were finished closely as we now see them when the 

 ice melted away, and hence they were of subglacial construc- 

 tion. 



The supposed manner of accumulation of drumlins may be 

 briefly sketched. It is well known that a stream of running 

 water will at one point carry along silt and sand that must be 

 dropped a little farther on where the current slackens, and the 

 bank thus begun grows slowly in a form of least resistance, at- 

 taining a maximum size when its increase of volume has so far 

 diminished the cross-section of the stream and consequently 

 increased the velocity that no more detritus can be dropped 

 there : but even then one end may be worn away while the 

 other grows, the adjustment of velocity to channel is not per- 

 manent. The motion of a glacial sheet has been justly com- 

 pared to that of a broad river. The comparison mav be ex- 

 tended so as to liken the active head-waters of a stream to the 

 presumably fast-moving part of the ice-sheet near its source or 

 center of dispersion where the greatest erosion srenerally takes 

 place. The delta of a river corresponds to the thinner and 

 slower- moving marginal area of an ice-sheet, where drift 

 brought from elsewhere is quietly and evenlv deposited, as in 

 Minnesota, and where erosion is relatively weak. A still fur- 

 ther agreement is discovered in comparing the drumlins and 

 sand-banks found in the middle course of the molten and solid 

 streams as suggested by the several authors quoted above. In 

 view of the irregularity of the surface on which the ice-sheet 

 moved and of the greater weakness of some rocks than others, 

 we must suppose an irregular velocity in the motion of the ice 

 and an unequal distribution of the rubbish beneath it. If the 



