The Flow of Glaciers. — Upham. 19 
of these varied agencies of stratification, it is doubtful if we can look 
with any confidence for criteria by which the annual snowfall can be 
safely distinguished from that of other periods. 
The original stratification could not have been very pronounced. 
Perhaps it was intensified somewhat during subsequent consolidation, 
but some new agency was necessary to produce the more definite part- 
ings and to introduce the layers of debris. This agency appears to 
have been a shearing movement between the layers. 
' Veined ok Ribboned Structure. 
Alternating blue and white ice lamina* approximately par- 
allel with the steep rock walls of valley glaciers, curving 
thence convexly downward and forward across the glacier, 
and streaming outward longitudinally along the line of union 
of confluent glaciers in parallelism with the medial moraines, 
were first noticed by Brewster in 1814, and were well described 
by Guyot in 1838, but were independently rediscovered by 
Forbes in 1841. The latter observer, in his monumental work, 
"Travels through the Alps of Savoy, ,: describes the develop- 
ment of the blue and white bands in nearly vertical planes, 
coinciding with differential upward shearing motion, where 
the ice within the space of a few hundred feet becomes rece- 
mented under much pressure at the base of a rapidly descend- 
ing and exceedingly crevassed part of a glacier. He accord- 
ingly decides that this veined or ribbon-like structure "is de- 
veloped during the progress of the ice downwards — is subject 
to the variations which its momentary conditions of constraint 
impress — and that it has not the slightest reference to the 
snow beds of the neve/" Having determined by measurements 
that the motion of the central part of the glacier surpasses 
that of its sides, which are hindered by the friction of the 
rock walls, Forbes endeavored to illustrate the shearing move- 
ments to which he attributed the veining or banding, as fol- 
lows :* 
There must be a solution of continuity between the adjacent parti 
cles of ice to enable the middle to move faster than the sides, [magine 
the surface of a glacier to be divided into a Dumber of stripes parallel to 
its length, and adjoining but not cohering. If it be ascertained that 
each stripe nearer the centre moves Taster than its neighbour Dearer the 
side, the stripes will move past one another parallel to their length, the 
central stripes gaining upon the lateral ones. If we attempt togive such 
a varying motion to the parts of a flat stiff body, as a long sheet of pa 
*Op. eit.. second edition. LS 1T>. p. 178. 
