PROGRESSION OF GLACIERS. 
253 
which join them. The beds on the left, which 
dip least, and are only folded gently downward, 
forming very open loops, are those of the Lauter- 
Aar, where the lateral pressure is comparatively 
slight. Those which are almost vertical belong 
in part to the several small trib- 
utary glaciers, which have been 
crowded together and very strong- 
ly compressed, and partly to the 
Finster-Aar. The close uniform 
vertical lines in this wood-cut 
represent a different feature in 
the structure of the glacier, called 
blue bands, to which I shall re- 
fer presently. These loops or 
lines dipping into the internal 
mass of the glacier have been the 
subject of much discussion, and 
various theories have been re- 
cently proposed respecting them. 
I believe them to be caused, as 
I have said, by the snow-layers, 
originally deposited horizontally, 
but afterwards folded into a more 
or less vertical position, in con- 
sequence of the lateral p 7 ^ assure 
brought to bear upon them. The 
sheets of dust and ol ice alter- 
nating with the snow-strata are 
