Prof. Huxley on the Structure of Glacier Ice. 257 
has been shown that they may be the mere result of the in- 
fluence of the motion of the glacier upon the form of any patch 
of dirt scattered accidentally upon its surface ; but besides these 
“dirt-bands,” the dirt on a glacier frequently presents a defi- 
nite arrangement upon a smaller scale, which zs connected with 
the minute structure of the glacier. We have both observed, 
for instance, in those parts of the Mer de Glace in which the 
structure is vertical, that the superficial layer of the wall of a 
crevasse is weathered into granules of tolerably even size and 
similar form. Nevertheless, dirt (or a coloured infusion) accu- 
mulates in larger proportion in those fissures which are parallel 
with the cleavage, and thus, from a little distance, the surface of 
the ice appears as if striated or ruled with lines parallel to the 
structure. The lines are separated by the width of the granules, 
and there may be several interposed between two blue veins. 
Why it is that those intergranular fissures which are parallel 
with the cleavage are the larger, is a question I will not for the 
present attempt to answer. It may be that the weathering takes 
place more rapidly in this direction, or it may be that these 
fissures being in the course of the flow of the water produced 
by the superficial waste of the ice, become enlarged more ra- 
pidly than the others. 
These markings, and the similar ones frequently to be ob- 
served on the upper surface of a glacier, might be termed 
“ dirt-lines,” to distinguish them from the great “ dirt-bands.”’ 
There is a third mode of arrangement of dirt, which, like the 
“ dirt-lines,” is dependent on the weathering of the ice, but the 
resulting strie are broad streaks, and not mere lines. These 
may perhaps be termed “ dirt-streaks.” 
I became acquainted with these quite recently, when, induced 
by Prof. Forbes’s description and representation of the “ struc- 
ture” of the glacier of La Brenva, I paid a visit to that glacier. 
Prof. Forbes states,— 
“The alternation of bluish-green and greenish-white bands 
which compose this structure, gives to this glacier a most beau- 
tiful appearance as seen from the mule-road. An attempt has 
been made in plate 5 to give some idea of this most charac- 
teristic display, and which is better seen here than in any other 
glacier whatever with which I am acquainted. The sketch was 
taken by myself from the point marked & on the map in July 
1842.”—Travels, 2nd Ed., p. 203. 
It must at once strike any one conversant with the ordinary 
character of the veined structure, that at the distance of the 
point on the mule-road from which Prof. Forbes’s view is taken, 
any veins of the usual dimensions must be totally invisible ; and 
I therefore approached La Brenva with the desire, if not the 
Phil. Mag. 8. 4. Vol. 14. No. 93. Oct. 1857. S 
