0. D. von Engeln — Sfaidies on Ice Structure. 471 



much lower and temperatures higher the flow was much more 

 rapid, indicates that with temperatures near the pressure melting 

 point the yield of the ice is much easier and also, as shown 

 by the experiments with the cubes, that flow once initiated 

 continues under diminished pressure. This latter principle 

 seems to be an important factor in that, applied to glaciers, it 

 explains the rapid waves of advance accompanied by a swelling 

 of the glacier tongues that have been noted in Alaska * and 

 elsewhere. 



A conception of the flow of glaciers as developed from the 

 results of the experiments and consideration of the constitution 

 and physical properties of glacier ice does not permit of 

 characterization as either plastic or viscous. It is a plastic 

 flow in the sense that the ice mass as a whole is permanently 

 deformed by the movement, but its component grains are not 

 subject or capable of such plastic deformation except in one 

 direction and this appears to be a minor factor. Much more 

 important is the movement that seems to be conditioned by 

 the presence of the interstitial film of low freezing liquid, 

 which may be characterized as viscous movement, by analogy 

 like that of a stiff cement-concrete mixture. 



From this point of view the glacier grains must be regarded 

 as the rock-fragment units of the concrete mixture and the 

 interstitial film as the liquid cement or lubricating substance 

 diminishing the inter-unit, hence internal friction between 

 them, thus facilitating movement analogous to that between the 

 molecules of a viscous solid. 



It may be questioned whether the volume of the interstitial 

 film can be sufficiently great to permit of the degree of 

 articulation between the ice grams required for continuous flow 

 without involving the distortion or destruction of the crystalline 

 units, hence preventing their progressive growth in size from 

 the upper to the lower portions of an ice tongue. In this 

 connection a calculation by Chamberlinf is interesting. He 

 figures that iu the interlocking, granular portion of a glacier 

 six miles long, with a movement of three feet per day, an 

 individual granule would need to move the length of its own 

 diameter, with reference to its neighbor, only once in thirty 

 years. On this basis it would appear that only a very slight 

 thickness of interstitial film in the liquid state would be required 

 to permit of all necessary readjustments between the crystal- 

 line grains, while keeping their interlocking structure intact and 

 permitting their growth in size. Where differential pressures 

 exist and an accelerated local flow is demanded this will be 



* Tarr, E. S. : Eecent Advance of Glaciers in the Yakutat Bay Eegion, 

 Alaska, Bull. Geol. Soc. Amer., xviii, pp. 257-286. 



f Chamberlin, T. C. : A Contribution to the Theory of Glacial Motion. 

 Decennial Publications of the Univ. of Chicago, Chicago. First series, vol. 

 ix, p. 201, 1904. 



