730 
ME. HOPKINS ON THE THEOET OE THE MOTION OF GLACTEES. 
in its total inability to account for the highly developed structural surfaces, their nearly 
vertical positions, and approximate perpendicularity to the axis of the glacier, at all 
points not far from its surface, and near to the bottom of an ice-fall (art. 56). It appears 
to me inconceivable that any physical or mechanical effect (such as the lamination in 
question) really and primarily due to the difference of motion of contiguous particles 
should not manifest itself, if seen at all, in those directions in which the actual difference 
of motion is greatest. Assuming the truth of this conclusion, the structural surfaces 
must necessarily be such as above represented (art. 56). The investigation of the forms 
and positions of these surfaces depends only on the recognized motion of the glacier, and 
geometrical reasoning in which there can be no ambiguity. Now superficial curves of 
structure at the bottom of an ice-fall, as thus theoretically determined, are extremely 
elongated loops (art. 56); whereas it is one of the best-established of glacial phenomena 
that the actual superficial lines of structure at the foot of an ice-fall are nearly straight 
and perpendicular to the axis of the glacier. I maintain, therefore, that it is altogether 
impossible that the differential theory can be true, unless the expression “ differential 
motion ” has a very different meaning from any which I have been able to attribute 
to it*. On the contrary, the pressure theory, so far as it asserts the perpendicularity 
of the structural surfaces to the lines of maximum internal pressure, appears to be in 
accordance with all the observations which have yet been made on the subject. 
Section VI. — On the Intensity of the Forces employed in dislocating and 
crashing the masss of a Glacier. 
60. We have seen that, according to the Pressure Theory, when a glacier is brought 
into a constraint which it can no longer resist, its structure, or the continuity of its mass, 
must be destroyed in one of the three ways already specified (art. 28). It would seem 
probable that this is frequently effected by the crushing of the mass, the structure being 
immediately restored by regelation. But here arises the question, Whence do the 
internal pressures and tensions derive sufficient intensity to produce these crushing 
effects 1 The enormous weight of the mass of a glacier at once presents itself as the 
cause of the required intensity. But suppose, after the glacier has been broken and 
crushed, it should be instantaneously restored to perfect continuity of crystalline struc- 
ture under the pressure to which, in any part, it may at the moment be subjected; 
why should the pressure, apparently the same as that under which it had, the instant 
before, regained its crystalline structure, again destroy the continuity of that structure % 
The highest mountain does not crush the strata which form its base. Again, the super- 
incumbent weight can only produce any great effect at depths sufficiently great, whereas 
the effects of the internal pressures and tensions are exhibited in the most superficial 
♦ "With respect to Principal Foebes’s conclusion, tliat a nearly horizontal force at the bottom of an ice- 
faU will produce a differential motion in nearly a vertical direction, I can only say that it appears to me so 
obviously opposed to the simplest conception of the action of such a force, as to be entirely inadmissible. 
It is unquestionably opposed to aU accurate mechanical investigation on the subject (art. 53). 
