On the Stratification of Vesicular Ice by Pressure. 465 



and 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 hubbies 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, ovtght 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") " loith 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 

 tlieory 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, 

 the difference of pressure in different directions must be continued 

 until a very considerable amount of this differential motion, or dis- 

 tortion, has taken place, to produce any sensible degree of stratifica- 

 tion in the vesicles. The absolute amount of distortion experienced 

 by any portion of the viscous mass is therefore an index of the per- 

 sistence of the differential pressure, by the continued action of which 

 the blue veins are induced. Hence also we see why blue veins are not 

 formed in any mass, ever so deej), of snow resting in a hollow or corner. 



As to the direction in which the blue vehis appear to lie, they 

 must, according to the theory, be something intermediate between 

 the surfaces perpendicular to the greatest pressure, and the surfaces 

 of sliding ; since they will commence being formed exactly perpen- 

 dicular to the direction of greatest pressure, and will, by the differ- 

 ential motion accompanying their formation, become gradually laid 

 out more and more nearly parallel to the sides of the channel through 

 which the glacier is forced. This circumstance, along with the com- 

 paratively weak mechanical condition of tlie white strata (vesicular 

 * Thirteenth Letter on CUaciers, seciioii (2), dated Dec. 1810. 



PhiL Mag. S. 4. Vol, 10. No. 109. Dec. 1858. 2 II 



