176 Notice of Prof. Forbes' s Travels in the Alps. 



tributed to this cause. The dirt and small stones are detained in those 

 wrinkles or curved lines, which are soft and oozy with water, and thus 

 the accumulated lines of dirt remain and exhibit a banded succession 

 of earthy materials. The author has elucidated and confirmed these 

 views by actual experiments made upon the flow of viscous and tena- 

 cious matters, which he finds to correspond with the great Alpine exhi- 

 bitions. Bishop Rendu, of Annecy, has insisted upon the plasticity of 

 the ice ; moulding itself to the endlessly varying forms and sections of 

 its bed, and he maintained that the centre of the ice stream moved the 

 fastest. Captain Hall says, " When successive layers of snow, often 

 several hundred feet in thickness, come to be melted by the sun and by 

 the innumerable torrents which are poured upon them from every side, 

 to say nothing of the heavy rains of summer, they form a mass not 

 liquid, indeed, but such as has a tendency to move down the highly in- 

 clined faces on which they lie, every part of which is not only well 

 lubricated by running streams resulting from the melting snows on every 

 side, but has been well polished by the friction of antecedent glaciers. 

 Every summer, a very slow but certain advance is made by these huge, 

 sluggish, and bulky, half snowy, half icy accumulations." 



Prof. Forbes justly remarks that no one has a right to assume the plas- 

 ticity of the materials of glaciers as the foundation of a theory, until 

 the fact has been proved by decisive observations and experiments. 

 " These observations have been made, and the result is, the viscous or 

 plastic theory of glaciers, as depending essentially on the three follow- 

 ing classes of facts 1 ' — first ascertained by himself in 1842, the proofs 

 being contained in the work now under consideration. For those de- 

 tails of proofs we have not room, but they are believed to sustain the 

 following conclusions : 



1. " That the different portions of any transverse section of a glacier 

 move with varying velocities, and fastest in the centre." 



2. " That those circumstances which increase the fluidity of a gla- 

 cier, namely, heat and wet, invariably accelerate its motion." 



3. " That the structural surfaces, occasioned by fissures which have 

 traversed the interior of the ice, are also the surfaces of maximum ten- 

 sion, in a semi-solid or plastic mass, lying in an inclined channel." 



The glaciers undergo a surprising depression during the summer, and 

 a correspondent elevation in the winter. In the summer, the streams 

 flowing beneath the glacier contribute to waste it ; the warmer earth 

 melts the inferior surface, and the lower extremity of the glacier, where 

 it touches the fields, moves faster than the upper, thus drawing it iout, 

 and thinning it mechanically. The difference of level between summer 

 and winter is sometimes twenty to twenty-five feet, and the depression 

 of summer is certainly made up in the winter and spring. This may 



