PROGRESSION OF GLACIERS. 
259 
looked the one and insisted upon the other as the 
prevailing feature, while that very feature has 
been absolutely denied again by others who have 
seen its fellow only, and taken that to he the 
prominent and important fact in this peculiar 
structural character of the ice. 
We have already seen how the stratification of 
the glacier arises, accompanied by layers of dust 
and other material foreign to the glacier, and 
how blue bands of compact ice may be formed 
parallel to the surface of these strata. We have 
also seen how the horizontality of these strata 
may be modified by pressure till they assume a 
position within the mass of the glacier, varying 
from a slightly oblique inclination to a vertical 
one. Now, while the position of the strata be- 
comes thus altered under pressure, other chan- 
ges take place in the constitution of the ice itself. 
Before attempting to explain how these chan- 
ges take place, let us consider the facts them- 
selves. The mass of the glacier ice is traversed 
by thin bands of compact blue ice, these bands 
being very numerous along the margins of the 
glacier, where they constitute what Dr. Tyndall 
calls marginal structure, and still more crowded 
along the line upon which two glaciers unite, 
where he has called it longitudinal structure. In 
the latter case, where the extreme pressure re- 
sulting from the junction of two glaciers has ren- 
