522 THE JOURNAL OF GEOLOGY. 



a. Of the drumlins, four sub-varieties may be recognized, and 

 it may prove serviceable to distinguish these and treat them as 

 distinct varieties until the mystery of the drumlin formation shall 

 be solved and the importance, or otherwise, of these distinctions 

 be determined. 



[a) Lenticular or elliptical hills. — These are the typical variety 

 of drumlins and consist of very remarkable 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. 

 The longer axis lies in the direction of glacial movement. This 

 is the most familiar form. 



(#) Elongated ridges.- — These have the same constitution as 

 the preceding and have similar terminal contours. The body 

 of the hill is, however, elongated to the extent of two or three 

 or occasionally several miles. These elongated ridges com- 

 monly lie parallel to each other, giving a markedly fluted char- 

 acter to the surface. They are thought worthy of being distin- 

 guished for the present, because the elongation of their forms 

 may prove a significant feature, and lead to the recognition of 

 some of the essential conditions of drumlin formation. 



(<r) Mammillary hills. — These have the same constitution as 

 the previous types, but differ from them in the extreme short- 

 ness of the axis. This, in some instances, is scarcely longer 

 than the transverse diameter. These are thought worthy of 

 being distinguished, because they emphasize more than either of 

 the preceding varieties the vertical element of the constructive 

 process. I know of nothing more extraordinary in glacial forma- 

 tions than the building up of these domes to the height of 

 50 to 60 or more feet with such steep sides and on so circum- 

 scribed and so nearly circular a base. There are no cases, so far 

 as I am aware, in which the base is strictly circular. There 

 seems always to be an element of elongation in the direction of 

 glacial movement. 



(cT) Till tumuli.- — These are low mounds of more than usually 

 stony material (so far as I have observed). They have not gen- 

 erally assumed the drumloidal curves of contour and profile, but 



