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variations of pressure in the solid in their neighbourhood, as to make 

 them, by melting and freezing, to extend, each obliquely towards the 

 other and from the parts of its boundary most remote from a plane 

 midway between them, perpendicular to the direction of pressure. 



The general tendency clearly is for the vesicles to become flattened 

 arid arranged in layers, in planes perpendicular to the direction of 

 the pressure from without. 



It is clear that the same general tendency must be experienced 

 even when there are bubbles of air in the vesicles, although no doubt 

 the resultant effect would be to some extent influenced by the run- 

 ning down of water to the lowest part of each cavity. 



I believe it will be found that these principles afford a satisfactory 

 physical explanation of the origin of that beautiful veined structure 

 which Professor Forbes has shown to be an essential organic pro- 

 perty of glaciers. Thus the first effect of pressure not equal in all 

 directions, on a mass of snow, ought to be, according to the theory, 

 to convert it into a stratified mass of layers of alternately clear and 

 vesicular ice, perpendicular to the direction of maximum pressure. 

 In his remarks "On the Conversion of the Neve' into Ice*," Pro- 

 fessor Forbes says, " that the conversion into ice is simultaneous" 

 (and in a particular case referred to "identical") " with the forma- 

 tion of the blue bands ; . . . . and that these bands are formed where 

 the pressure is most intense, and where the differential motion of the 

 parts is a maximum, that is, near the walls of a glacier." He farther 

 states, that, after long doubt, he feels satisfied that the conversion of 

 snow into ice is due to the effects of pressure on the loose and porous 

 structure of the former ; and he formally abandons the notion that 

 the blue veins are due to the freezing of infiltrated water, or to any 

 other cause than the kneading action of pressure. All the observa- 

 tions he describes seem to be in most complete accordance with the 

 theory indicated above. Thus, in the thirteenth letter, he says, 

 " the blue veins are formed where the pressure is most intense and 

 the differential motion of the parts a maximum." 



Now the theory not only requires pressure, but requires difference 



of pressure in different directions to explain the stratification of the 



vesicles. Difference of pressure in different directions produces the 



"differential motion" referred to by Professor Forbes. Further, 



* Thirteenth Letter on Glaciers, section (2), dated Dec. 1846. 



