146 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
ence of velocity of the centre and sides is greater near the lower extremity of the 
glacier, and that the velocity is more nearly uniform in the higher part ; this corre- 
sponds to the less elongated forms of the loops in the upper part of the glacier. 
4th. In the highest part of such glaciers, as the curves become less bent the structure 
also vanishes. 5th. In the wide saucer-shaped glaciers which descend from moun- 
tain slopes, the velocity being as in shallow rivers nearly uniform across their breadth, 
no vertical structure is developed. On the other hand, the friction of the base de- 
termines an apparent stratification, parallel to the slope down which they fall. 6th. It 
also follows immediately (assuming it as a fact very probable, but still to be proved, 
that the deepest part of the glacier moves slower than the surface) that the frontal 
dip of the structural planes of all glaciers diminishes towards their inferior extremity, 
where it approaches zero, or even inclines outwards, since then the whole pressure of 
the semi-fluid mass is unsustained by any barrier, and the velocity varies (probably 
in a rapid progression) with the distance from the soil ; whilst nearer the origin of the 
glacier the frontal dip is great, because the mass of the glacier forms a virtual barrier 
in advance, and the structure is comparatively indistinct, for the same reason that 
the transverse structure is indistinct, viz. that the neighbouring horizontal prisms of 
ice move with nearly a common velocity. 7th. Where two glaciers unite, it is a fact 
that the structure immediately becomes more developed. This arises from the in- 
creased velocity, as well as friction of each due to lateral compression. 8th. The 
veined structure invariably tends to disappear when a glacier becomes so crevassed 
as to lose horizontal cohesion, as when it is divided into pyramidal masses. Now 
this immediately follows from our theory ; for as soon as lateral cohesion is destroyed, 
any determinate inequality of motion ceases ; each mass moves singly, and the 
structure disappears very gradually*.” 
In explaining the theory of the veined structure at a meeting of the Royal Society 
of Edinburgh on the 20th of March 1843, I stated that I had arrived at the conclu- 
sion that crevasses resulting from tension in certain parts of a glacier, must be 
formed at right angles to the surfaces of discontinuity or structural veins where they 
intersect the surface: a law conformable to the empirical one discovered by me on 
the glacier of the Rhone in 1841^, since generalized in other cases, and which even 
the adversaries of my theoretical views have admitted to be a correct statement of 
the facts 
My attention was at that time (March 1843) turned by my learned and acute 
friend Mr. W. A. Cadell, to the veined structure of the slag of iron furnaces as due 
to the difference of velocity of the parts producing surfaces of separation and peculiar 
molecular condition. The transition was easy to the case of volcanic rocks and lava 
* Third Letter on Glaciers, Edinburgh Philosophical Journal, October 1842. 
f Edinburgh Philosophical Journal, January 1842. 
+ Bibliothhque Universelle, tome xliv. p. 153. “ C’est en effet un fait assez general que les bandes bleues 
coupent a angle droit les crevasses,” &c. 
