ME. HOPKINS ON THE THEOET OE THE MOTION OF GLACIEES. 
717 
glacier. In the first case, especially if the inclination of the valley should gradually 
increase and become considerable, the transverse pressure, B, may be much increased, 
and the longitudinal pressure. A, much diminished as we descend the valley, till it 
becomes less than B. In such case the curves of structure (so far as they depend alone 
on the action of the external forces) would become more and more elongated, as repre- 
sented approximately in fig. 9. If B should become greater than A, 
. o 2F 
tan ^5 
Fig. 9. 
and the case becomes the same as that previously considered (art. 43), in which the 
formation of the longitudinal structure is explained. So long, however, as A remains a 
pressure sufiicient to produce the laminar structure at right angles to the axis, the loops 
will be completed ; if A become too small to produce the structure, the curves will not 
be continued across the axis, and the structure will exist only as a marginal structure ; 
and if B still increase (by the glacial valley becoming very narrow), so as to produce 
the longitudinal structure along the axis, the structure will become longitudinal, as 
represented by the two lower lines of structure in fig. 9. 
46. In the second case above mentioned, of a rapidly divergent valley of small incli- 
nation, the mass will be urged onwards by a great radial force, as above explained 
(art. 30), and the curves of structure will be perpendicular to the crevasses, and conse- 
quently approximately concentric about the foot of the ice-fall, where this structure 
commences. 
47. Application of the preceding Theoretical Besults . — It must be recollected that the 
phenomena of glacial structure as they actually exist at any moment, are not merely the 
results of the instantaneous action of the mechanical and physical causes to which their 
original formation may have been due, but to those causes together with the effect pro- 
duced by the motion of the glacier. In the formulae of this section I have entirely 
neglected the influence of transmission by this motion, on the forms of the curves of 
structure, and have spoken of them as though they were dependent only on the forces 
originally producing them, and unaffected by the unequal motions of the central and 
marginal portions of the glacier. This, however, cannot be the case, unless we suppose 
the structure to be modified at every instant in exclusive obedience to the physical and 
mechanical causes in which it originates- — a supposition having a great apparent impro- 
bability. The motion of a glacier being known, it is merely a geometrical problem to 
MDCCCLXII. 5 F 
