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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 deep, of snow resting in a hollow or corner. 



As to the direction in which the blue veins 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 the white strata (vesicular 

 layers between the blue strata), must, I think, make these white strata 

 become ultimately, in reality, the surfaces of " sliding" or of " tear- 

 ing," or of chief differential motion, as according to Professor 

 Forbes' s observations they seem to be. His first statement on the 

 subject, made as early as 1842, that " the blue veins seem to be per- 

 pendicular to the lines of maximum pressure," is, however, more in 

 accordance with their mechanical origin, according to the theory I 

 now suggest, than the supposition that they are caused by the tear- 

 ing action which is found to take place along them when formed. 

 It appears to me, therefore, that Dr. Tyndall's conclusion, that the 

 vesicular stratification is produced by pressure in surfaces perpendi- 

 cular to the directions of maximum pressure, is correct as regards the 

 mechanical origin of the veined structure ; while there seems every 

 reason, both from observation and from mechanical theory, to accept 

 the view given by Professor Forbes of their function in glacial motion. 



The mechanical theory I have indicated as the explanation of the 

 veined structure of glacial ice is especially applicable to account for 

 the stratification of the vesicles observed in ice originally clear, and 

 subjected to differential pressure, by Dr.Tyndall ; the formation of the 

 vesicles themselves being, as remarked in my last letter *, anticipated 

 * See Proceedings for February 25, 1858. 



