258 THE JOURNAL OF GEOLOGY. 



were competent to traverse very considerable depths of ice with- 

 out being entirely absorbed, being chiefly waves of short vibra- 

 tion. Those who have been beneath glaciers have observed that 

 the amount of transmitted light is not inconsiderable. The 

 transmitted rays of short vibration, so far as they reached the 

 bottom, were arrested and, by transformation to rays of longer 

 vibration, brought to bear upon the base of the glacier. The 

 basal wastage from this source may be presumed to have 

 increased somewhat in proportion as the ice thinned towards its 

 margin, but this would be offset to some degree by a probable 

 increase of surface detritus that would cut off the rays. The 

 combined effect of these agencies would appear to have been 

 not inconsiderable. 



(5) In any vertical section of a glacier the lagging of the 

 basal portion causes the plane of the section to lean forward, 

 which means that each part is brought nearer to the bottom, car- 

 rying with it whatever material is enclosed. This being a gen- 

 eral phenomenon justifies the conclusion that the tendency of the 

 ice of the interior of a glacier is to flow obliquely forward and 

 downward. Exceptions to this may be found when the resist- 

 ance of a given portion of the base is greater than that of the 

 portion immediately in its rear, in which case the latter may tend 

 to flow over the former, but this will be reversed when the ratio 

 of resistance is changed and would be, at most, but a variation 

 of action, not a general law of action. 



The combined effect of all these agencies was, if I reason cor- 

 rectly, to bring back to the base of the ice any material that 

 owing to the special causes named, or to others, had been forced 

 up into the lower parts of the ice. They tended to preserve the 

 basal character of whatever had once become basal. And this 

 seems to be supported by observations on existing glaciers. 



These considerations have a very specific bearing upon the 

 horizon at which drumlins, osars (eskers), and kames were 

 formed. These all contain large quantities of local material, of 

 basal derivation. If the view here stated is correct, these must 

 be very strictly basal deposits, in general. There are doubtless 



