PROGRESSION OF GLACIERS. 
261 
gin of these secondary bine bands is the true one. 
lie suggests that layers of water may be formed 
in the glacier at right angles with the pressure, 
and pass into a state of solid ice upon the re- 
moval of that pressure, the pressure being of 
course relieved in proportion to the diminution 
in the body of the ice by compression. The num- 
ber of blue bands diminishes as we recede from 
che source of the pressure, — few only being 
formed, usually at right angles with the surfaces 
of stratification, in the middle of a glacier, half- 
way between its sides. If they are caused by 
pressure, this diminution of their number toward 
the middle of the glacier would be inevitable, 
since the intensity of the pressure naturally fades 
as we recede from the motive power. 
Dr. Tyndall also alludes to another structure 
of the same kind, which he calls transverse struc- 
ture, where the blue bands extend in crescent- 
shaped curves, more or less arched, across the 
surface of the glacier. Where these do not co- 
incide with the stratification, they are probably 
formed by vertical pressure in connection with 
the unequal movement of the mass. 
With these facts before us, it seems to me plain 
that the primitive blue bands arise with the strat- 
ification of the snow in the very first formation 
of the glacier, while the secondary blue bands are 
formed subsequently, in consequence of the on- 
