1846.] OBJECTIONS TO PLASTICITY CONSIDERED. 159 



falling upon it, but rather to the prolongation of the winter cold 

 into spring and summer, which causes the condensing or accu- 

 mulating process to be in excess, and therefore the thickness of 

 the plastic mass to accumulate beyond its due amount. 



Thus we have the following phenomena, all independently 

 observed, reconciled and explained by one hypothesis ; the 

 general convexity of the crevasses upwards, notwithstanding the 

 excess of motion in the centre ; the general vertically of the 

 crevasses, notwithstanding the retardation of the bottom ; the 

 perfect state of the crevasses every spring succeeding their 

 visible collapse in autumn ; the ascertained velocity of different 

 parts of the glacier, and the diversity of the annual changes 

 which these velocities present ; the seemingly opposed facts 

 showing the glacier to be subjected to powerful tension, pro- 

 ducing crevasses, and yet to be under a compression which pro- 

 duces in some places the frontal dip ; and finally, the renewal 

 of the level of the ice during winter, which has been lost partly 

 by superficial melting, but as much or more so by the attenua- 

 tion and collapse of the glacier during summer. These various 

 effects of one cause, though they do not embrace all the pheno- 

 mena of glaciers, certainly include a very remarkable and com- 

 plicated group of facts. 



Plasticity Veined Structure. I certainly never expected, 

 when promulgating the viscous theory, that it would have met 

 with so much opposition on the ground that the more familiar pro- 

 perties of ice are opposed to the admission of its plasticity ; and 

 that the fragility of hand specimens should be considered as 

 conclusive against the plastic effect of most intense forces acting 

 on the most stupendous scale upon a body placed in circum- 

 stances which subject it to a trial, beneath which the most mas- 

 sive constructions of the pyramid-building ages would sway, 

 totter, and crumble. In an age when generalizations of the 



on the rock in 1842 were concealed : and he attributes this, apparently with reason, 

 to the extreme lateness and coldness of the spring. [See also the observations on 

 the glacier of La Brenva in the Twelfth Letter below.] 



