ORIGIN OF THE VEINED STRUCTURE OF ICE. 255 



moves in such a fashion cannot, in any true sense of the word, 

 be termed a rigid solid, and must be granted to be ductile, 

 viscous, plastic, or semifluid, or to possess qualities represented 

 by any of these terms which we may choose to adopt as least 

 shocking to our ordinary conception of the brittleness of ice.* 



Origin of the Veined Structure of Ice. Any mechanical 

 theory of glaciers must be more or less imperfect which does 

 not explain the remarkable veined or ribboned structure of the 

 ice, which, with its peculiar course through the interior of the 

 glacier, has been described at page 245. In applying the 

 theory of quasi-fluid motion to explain this structure, two great 

 difficulties are experienced ; first, our confessed ignorance of 

 the modus operandi of those molecular forces which induce 

 pervading structures of this kind ; secondly, our imperfect 

 acquaintance with the laws of motion of semifluids under the 

 action of gravity, even in simple cases. Nevertheless it seems 

 possible to prove, partly from admitted mechanical principles, 

 partly from direct experiments, that the tendency of the motion 

 is to produce such a structure. 



The fundamental idea is this, that the veined or ribboned 

 structure of the ice is the result of internal forces, by which 

 one portion of ice is dragged past another in a manner so 

 gradual as not necessarily to produce large fissures in the ice, 

 and the consequent sliding of one detached part over another, 

 but rather the effect of a general bruise over a considerable 

 space of the yielding body. According to this view, the deli- 

 cate veins seen in the glacier, often less than a quarter of an 

 inch wide, have their course parallel to the direction of the 

 sliding effort of one portion of the ice over another. Amongst 

 other proofs of this fundamental conception that the veined 



* For a fuller reply to the objections which have heen urged against the theory 

 of the plasticity of glaciers, see Philosophical Transactions, 1846, particularly pp. 

 162, etc. [and p. 140, etc., of this volume]. The confirmatory observations of MM. 

 Agassiz and Schlagintweit on other glaciers, and their adoption of views virtually 

 the same with those maintained above, have proved convincing to a majority of 

 those who at first rejected a theory apparently opposed to commonly received 

 notions. See Mousson, Die GUtscher der Jetzt-zeit, who says, p. 162, speaking of 

 the plastic theory, " Er steht noch heute unangefochten da." 



