212 T. C. CHAMBERLIN — GLACIAL STUDIES IN GREENLAND. 



according to the testimony of all observers, pervades the interior and 

 basal portions of glaciers, has, with little doubt, mainly descended from 

 above. 



We seem, therefore, altogether safe in repeating that every warm day 

 sends down into the glacier a wave of heat-energy, sensible or potential, 

 and that every night sends after it a wave of reverse nature. These waves 

 follow each other indefinitely, until by intercurrent agencies they become 

 vanishing quantities. Each season sends through the mass a greater and 

 more complex wave. The problem, therefore, in simplified form, postu- 

 lates a mass of ice-granules predisposed to melt at certain points and to 

 freeze or to promote freezing at others, acted upon by the ever present 

 but differential force of gravity and swept by successive waves of heat- 

 energy competent to cause melting where predisposition to melting exists 

 and to cause growth by freezing where predisposition to freezing exists. 

 Out of this it would seem that localized freezings and thawings, growths 

 and decadences, innumerable and constantly changing, must result, and 

 with them motion of the granules themselves and of the common mass. 

 This statement lacks ver}^ much in completeness and qualification, and 

 I can only ask you to accept it as indicating the line of thought to which 

 the observations of the summer have led. 



If the truth lies along this line, it is obvious that these evolutions 

 would proceed with different rapidity in different portions, and that they 

 might affect an individual layer in a degree different from its neighbor 

 layer, or they might affect the common mass to a nearly equal degree, 

 and that therefore differential movements, alike with common move- 

 ments, would be possible under suitable conditions, and that gravity 

 would control the whole mass much as if it were a liquid. 



Viscosity.^ — My observations seem to be adverse to anything which can 

 be properly termed viscous fluency. On two or three of the glaciers it 

 was observed that the surface rises in the direction of the movement of 

 the ice, so that the surface streams flow backward. Possibly this may 

 be explained on the basis of a viscous flowage of the mass, but it seems 

 much more consonant with the view that the ice-mass was pushed for- 

 ward by its own internal molecular changes, and that it rode up over 

 the inequalities of its bottom as any flexible but relatively rigid sheet 

 would do. 



*The term " viscosity " unfortunately has two senses which are nearly contradictory. Both are 

 derived from the original use of "viscous " to signify a sticky, gelatinous, tenacious, semifluid 

 substance, such as the exudation or extract from the sap of the genus viscum. In one case atten- 

 tion is fastened on the pliancy or semiflnidity; in the other, on the adhesiveness or tenacity. In 

 the first ease viscosity becomes opposed to rigidity and implies an element of fluidity; in the 

 other it only needs to be indefinitely increased to become identical with rigidity, infinite viscosity 

 being perfect rigidity. The term is commonly used in glacial discussions to signify a degree of 

 fluidity, while in physical investigations it more commonly means a degree of tenacity. 



