PROGRESSION OF GLACIERS. 
255 
nomena of the glacier are not to be seen at all 
times, nor under all conditions. During the first 
year of my sojourn on the glacier of the Aar, I 
was not aware that the stratification of its tribu- 
taries was so universal as I afterward found it to 
be ; the primitive lines of the strata are often so 
far erased that they are not perceptible, except 
under the most favorable circumstances. But 
when the glacier has been washed clean by rain, 
and the light strikes upon it in the right direc- 
tion, these lines become perfectly distinct, where, 
under different conditions, they could not be dis- 
cerned at all. After passing many summers on 
the same glacier, renewing my observations year 
after year over the same localities, I can confi- 
dently state that not only do the lines of stratifi- 
cation exist throughout the great glacier of the 
Aar, but in all its tributaries also. Of course, 
they are greatly modified in the lower part of the 
glacier by the intimate fusion of its tributaries, 
and by the circumstance that their movement, 
primarily independent, is merged in the move- 
ment of the main glacier embracing them all. 
We have seen that not only does the centre of a 
glacier move more rapidly than its sides, but that 
the deeper mass of the glacier also moves at a 
different rate from its more superficial portion. 
My own observations (for the details of which 
T would again refer the reader to my “ Systeme 
