250 Prof. Huxley on the Structure of Glacier Ice. 
towards the surface, and not into the depth of the ice, I am in- 
clined to think that these cracks were produced in cutting the 
ice to thin away the outer wall of the cavity. 
I repeated these experiments in the neighbourhood of the 
Grand Moulin; on the Moraine du Noire, somewhat higher than 
the Couverele ; and on different parts of the Glacier du Géant, 
and everywhere with similar results. Furthermore, having 
carefully bored a vertical hole in the deep ice of the Mer de 
Glace, opposite the Montanvert, I filled it with the infusion, and 
having covered over the aperture with a roof of ice-blocks, I left 
it until the next morning. It rained hard during the night ; 
and on revisiting the spot after an interval of about fifteen 
hours, I found that the covering blocks had slipped off, and that 
the liquid occupied only about the lower two-thirds of the cavity. 
No trace of infiltration could be discovered ; but the lower part 
of the cavity had changed its figure from cylindrical to irregular 
and botryoidal. I conceive that the sinking of the fluid must 
be accounted for by the enlargement of the cavity consequent 
upon this botryoidal excavation of its walls ; and I suppose that 
the ice-blocks proving an insufficient shelter, the rain poured 
into the hole, and keeping up a constant supply of comparatively 
warm liquid, eroded its walls in the way described. However 
this may be, the fact that the liquid had produced a fresh sur- 
face for itself, is important, as it shows that the absence of in- 
filtration through the veins intersected by the cavities containing 
the coloured infusion is not dependent on a condensation of 
their walls by the auger. 
To eliminate any error of this kind, however, I took a small 
block of the deep ice, and with a sharp knife fashioned it into a 
cup, whose walls varied in thickness from 4 to 2rds of an inch. 
Filled with the infusion and surrounded with ice, this cup re- 
mained for two hours without showing a trace of infiltration 
along its structural planes. 
I can only conclude from these experiments, that the chief 
substance of a glacier is as essentially impermeable as a mass of 
marble or slate ; and that though it may be traversed here and 
there by fissures and cracks, these no more justify us in speaking 
of glacier ice as “ porous,” than the joints and fissures in a 
slate quarry give us a right to term slate porous. We do not 
call iron porous because water runs out of a cracked kettle. 
The extreme porosity of what I have termed the “ superficial 
layer,” however, is no less certain, and masmuch as this layer 
is continually and rapidly wasting away at its surface, it must 
be as constantly reformed from the solid glacier ice beneath. 
The fact observed by Professor Agassiz, that under a moraine 
(that is, where covered and protected by stone and gravel) the 
