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ME. HOPKINS ON THE THEOET OE THE MOTION OF OLACIEES. 
producing them, it is evident that it will generally be difficult or impossible to recog- 
nize by observation the modification which may have been produced in them by trans- 
mission. The case of structure represented in fig. 9 (art. 45), if the real structure 
in a glacier were sufficiently developed and carefully observed, would aflbrd a test as to 
whether the forms were original, or had been modified by transmission; for, in the 
latter case, the loops would never change by elongation into an absolutely longitu- 
dinal structure as represented in the figure referred to. But this test is far, at present, 
from being a practical one. Again, in the glacier of the Aar, if the structure were per- 
fectly developed, it would afford us the required test, as is easily seen by comparing 
fig. 17 and its completed loop in the main glacier, as transmitted from the tributary, 
with fig. 11 and the lines of longitudinal structure in the main glacier. If the glacier 
exhibited either of these structures complete, it would testify at once to the point in 
question. I am not aware, however, that the structure, to whatever cause it may be 
due, is sufficiently developed, or has been observed with sufficient care, to afford any 
positive testimony on the subject. The glacier of the Khone, also, fails to afford us a 
practical test on this point; for the curves of structure may there be accounted for 
either by supposing them instantaneously formed or transmitted fi:om the fall where 
they originate. 
The supposition that the structural surfaces, at points of a glacier remote from the 
place at which they commence, are merely the effects of transmission, involves the 
conclusion that the veined structure can only be originally formed under enormous 
pressure (according to the pressure theory), like that at the bottom of an ice-fall, and 
that, when once formed, it is difficult to obliterate. In this case it would seem that the 
transverse curves of structure formed at the bottom of a fall ought to be distinctly pre- 
served at a distance from it, as elongated loops extending completely across the glacier. 
I am not aware how far they are so, either in the case in which they proceed from the 
Talefre fall, for instance, on the Mer de Glace, or from that of the Geant. The glacier 
of La Brenva above cited (art. 49) seems to afford an example of the instantaneous 
generation of the veined structure under a pressure which may not be comparable with 
that immediately below the junction of the Aar tributaries ; and this same example at 
La Brenva seems also to afford an instance of a comparatively sudden obliteration of the 
structure where the immediate cause of it ceases to act. We may also remark that the 
marked and increased development of the longitudinal lamination along the central 
moraines proceeding from the junction of two considerable glaciers, appears to indicate 
the local efficiency and instantaneous effect of the causes of the structure. But par- 
ticular cases of this kind require to be examined with greater accuracy of detail before 
we can derive from them any reliable inferences. It would seem most probable that the 
structure in any particular locality may frequently be due partly to the instantaneous 
action of physical causes, and partly to transmission. The question well deserves the con- 
sideration of those glacialists who may interest themselves about this curious structiure 
of glacial ice, and the physical and mechanical causes to which it may be due. 
