718 ME. HOPKINS ON THE THEOET OE THE MiOTION OE GLACEEES. 
point out accurately the nature of the modification which would be produced in a given 
curve or surface of structure by transmission alone ; the amount of that modification 
can only be known by observation. I shall investigate this subject in a subsequent sec- 
tion. At present, in the application of the preceding formul8e to particular cases, my 
conclusions will be restricted to the hypothesis of the structure being due alone to the 
instantaneous action of the forces tending to produce it in each particular locality. 
48. The marginal structure is found more or less developed in most canal-shaped 
glaciers. It has been best observed, perhaps, on the glacier of the Aar and its tributa- 
ries, and on the glaciers at Chamouni. It sometimes coexists with marginal crevasses. 
In such cases the curves of structure ought to be, according to the pressure theory, 
perpendicular to the crevasses. Moreover, the coexistence of the structm'e and cre- 
vasses implies a longitudinal tension and a transverse pressure ; in which case the 
direction of greatest tension will make an angle of much less than 45 with the axis or 
sides of the glacier, and such, therefore, will also be the case with the marginal curves 
of structure. This seems accordant with the best observations ; for though the forms 
and positions of these curves have not yet been observed with all the care they require, 
there is little doubt as to their meeting the sides of the glacier at finite angles. The 
law of perpendicularity between the structural curves and the crevasses was first 
observed, I believe, by Principal Foebes on the glacier of the Aar ; and in speaking 
of the Talefre glacier* he remarks, “The crevasses as they present themselves are 
convex towards the origin of the glacier, and here, as in other cases, perpendicular to 
the veined structure.” The same impression appears to have been produced on other 
observers. 
49. It is manifest that the marginal structure may be modified by any local con- 
ditions which afiect the internal pressure of the glacier. Principal Foebes describes 
what he regards as an extraordinary case, and one to which he appeals as afibrding a 
crucial test in favour of his own views 'f. It is furnished by the glacier of La Bren\a, 
and is represented by fig. 10. The line z C B D represents the side of the glacial valley, 
Eig. 10. 
:e 
in which projections at B and D appear to form a somewhat sudden obstacle to the 
onward motion of the glacier in the direction FD. Now it is certain that the maxi- 
* Occasional Papers, p. 192. t IMd. p. 56. 
