232 THE ICE AGE W NORTH AMERICA. 



rectly represent the conditions and movements of this at still 

 greater depths. If this is correct, and I believe it is, it follows 

 that snch fragments of rock are not rigidly held in fixed posi- 

 tions in the under surfaces of glaciers and carried irresistibly 

 along at the same rate, but that the constantly melting ice 

 actually flows over them, and that their motion is one of ex- 

 treme slowness, even when compared with the motion of the 

 glacier itself.* 



In a visit to the glaciers of Norway in 1886, Professor J. 

 W. Spencer found abundant confirmation of Professor Giles's 

 inferences concerning the low eroding power of glaciers in 

 certain conditions. He reports that the advancing Nor- 

 wegian glaciers '* do not conform to the surfaces over which 

 they pass, but are apt to arch over from rock to rock and 

 point to point, especially as they are descending the ice-falls. v 



Professor Spencer continues : 



Beneath the glaciers of Fondal, Tunsbergdal, and Buardal, 

 in the northern, north-central, and south-central snow-fields 

 of Norway, as well as under other glaciers, I observed many 

 stones inclosed in ice resting upon the rocks, to whose surfaces 

 — sometimes flat, sometimes sloping steeply — they adhered by 

 friction and by the pressure of the superincumbent weight. 

 Although held in the ice on four sides with a force pushing 

 downward, the viscosity of the ice, or the resistance of its 

 molecules in disengaging themselves from each other in order 

 to flow, was less than that of the friction between the loose 

 stones and the rock ; consequently, the ice flowed around and 

 over the stones, leaving long grooves upon the under surfaces 

 of the glacier. 



An example of the ability of the ice to flow like a plastic 

 body was shown in a cavern four hundred feet higher than the 

 end of the glacier, where the temperature was 4° C, while that 

 outside was 13° 0. Upon the debris of the floor rested a 

 rounded bowlder, whose longer diameter measured thirty inches. 

 A tongue of ice, in size more than a cubic yard, was hanging 



* "American Journal of Science," vol. cxvi, 1878, p. 366 et seq. 



