726 
ME. HOPKESTS ON THE THEOEY OF THE MOTION OF GLACIEES. 
will be comparatively large near the sides of the glacier. The superficial curves of 
structure would be all straight lines parallel to the axis of the glacier. 
In actual glaciers, the forms of the curves of structure on the outer surface would be 
modified by the fact of the unequal thinning of the glacial mass in consequence of the 
more rapid thawing towards its lower extremity, so that the existing external surface is 
no longer parallel to the longitudinal lines of motion. Consequently the outer surface 
would intersect the internal structural surfaces obliquely, and the superficial curves of 
structure would be changed from straight parallel lines to very elongated loops, but 
would never approximate to straight transverse lines, as they always do at the foot of an 
ice-fall. We may also observe that there could be no longitudinal development of veins 
along the axis of the glacier, as the immediate result of difierential motion. Such veins 
could only exist by transmission, as will be explained in the next section, where I shall 
revert to the forms of the surfaces of greatest differential motion which have been here 
investigated. 
57. Modification of the Veined Structure arising from the Motion of the Glacier . — I now 
proceed, according to my intention as above expressed (art. 47), to examine the modifica- 
tions produced by the general motion of a glacier, on the forms and positions of the lines 
and surfaces of structure as originally produced by the immediate action of the causes to 
which they may be due. Such modifications as can be observed on the surface of a gla- 
cier, are principally due to the more rapid motion of its central portion ; the interior 
surfaces of structure must also be modified by the more rapid motion of the upper sur- 
face of the mass. In an ordinary canal-shaped glacier, it is manifest that the marginal 
curves of structure will thus be brought into more approximate parallelism with the sides, 
if originally inclined to them. This modification will be small. A greater one will be 
produced in the transverse structure, such as is usually found at the foot of an ice-fall. 
There the superficial curves of structure run almost directly across the glacier, by the 
motion of which they will be transformed into elongated curves, at distances from the 
fall sufiiciently great, the elongation increasing with that distance. In a glacier, hke 
the Aar, formed by the junction of two great tributaries, the modification of structure 
below their confiuence will be more complicated. Let fig. 17 represent the glacier as 
Fig. 17. 
