498 Mr. J. Ball on the Structure of Glaciers. 
make the upper surface somewhat convex. Any one who has a 
difficulty in following the description may easily construct a 
miniature model, by doubling together fifteen or twenty thick- 
nesses of leather or strong paper, and then cutting through the 
fold in a direction slightly inclined to it. On paring the edges 
of the new section with a sharp knife, it will be found that the 
arrangement of the lines exposed on the upper face of the mo- 
del, and the inclination to its surface of the lamine of leather or 
paper, very closely correspond to the ordinary disposition of the 
veined structure in glaciers that have flowed through a narrow 
valley. If it be objected that, as the hypothesis supposes that 
several separate streams, each with its own system of veined 
structure, were pressed together to make up the main glacier, 
the terminal section presented by the front of the glacier should 
present an equal number of systems of shell- or spoon-shaped 
structure, the answer may be found in observations detailed by 
M. Agassiz, and represented on his map of the Lower Aar gla- 
cier, which have the advantage of being supported by a great 
degree of anterior probability. Each separate ice-stream that 
goes to make up the glacier can subsist only so long as the 
supply of ice from its source meets the waste that arises during 
the process of ablation. It is not likely that any two of the 
streams that supply the glacier are exactly alike in this respect : 
one will bring with it a more abundant supply than the other, 
and survive the longest. Hence that one will by itself consti- 
tute the extreme front of the glacier, its structural surfaces will 
alone be perceived, and little if any trace of the other affluents 
will there be visible. 
By analogous considerations the position of the veined struc- 
ture may in most cases be explained; but there is one point 
which it has always seemed to me difficult to reconcile with the 
stratification theory,—I allude to the increased dip wards of 
the constituent veins, which is constantly observed in ascending 
the frontal portion of glaciers. This disposition is common to 
the great glaciers which have flowed through long channels, and 
to smaller glaciers of the second order that have moved but very 
slowly in depressions upon the flanks of the higher Alps. In 
particular cases the pressure theory appears to be more competent 
to account for it, but it seems to me that a complete and satis- 
factory explanation is yet to be sought for. 
The stratification theory is in no way inconsistent with the 
occasional occurrence of “ lenticular structure” in the fragments 
of blue ice, or the more common appearance of thinning out in 
the blue veins. The unequal advance of different portions of 
the glacier cannot take place without frequent disruption of the 
surfaces of blue ice, and though these may again be united so as 
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