Geological Papers. 91 



tations of phenomena that are difficult to explain, (c) The 

 excess of subglacial drift in drumlins opposes the interglacial 

 theory of their accumulation. 



The drumlin investigations and arguments given above 

 seem to show that drumlins were formed on the protected 

 side of some projecting hard substance which resisted the 

 wear and erosion of the ice. They seem to have collected in 

 a way similar to the way snow drifts behind a wind obstruc- 

 tion, except that it was closed in at the top and on the sides 

 by ice. Drumlins being more numerous in mountain regions 

 where there would be immovable obstacles to the ice advance, 

 over which the ice must pass, seem to bear out this conclusion. 

 A drumlin may therefore be defined as a lenticular, compact, 

 unstratified mound or hill having its longer axis parallel with 

 the direction of local glacial striation. 



Drumlin Varieties. — Mr. Chamberlin recognizes four sub- 

 varieties of drumlins, as follows : 



Lenticular or Elliptical Hills. — These consist of very re- 

 markable aggregations of till in hills of dolphin-back form 

 whose longer axes are two or three, or at most a few times 

 longer than their transverse diameters. Their longer axes lie 

 in the direction of the glacial movement. 



Elongated Ridges. — These have the same constitution as the 

 preceding and have similar terminal contours. They differ 

 from the above principally in the fact that they are elongated 

 more, their length often exceeding three miles. 



Mammilary Hills. — These have the same constitution as the 

 previous types, but differ from them in the extreme shortness 

 of the axis, the axis often being scarcely longer than the 

 transverse diameter. Though nearly round in form they are 

 always elongated in the direction of glacial movement. 



Till Tumuli. — These are low mounds of more than usually 

 stony material, as a rule. They have not generally assumed 

 the drumloidal curves of contour and profile, but their nature 

 is such as to have suggested that they are the immature nu- 

 clei of drumlins. Their genesis, however, is in doubt. 



DEPOSITS THAT ARE SOMEWHAT DRUMLOIDAL IN FORM. 



Crag and Tail. — These embrace the well-known accumula- 

 tions of till in the lee of rocky crags or embossments. 



Pre-crag. — These embrace the less well-recognized accumu- 

 lations of till in the lee of rocky crags. 



