ON THE STRUCTURE OF GLACIER ICE 49 1 



cases in any other way. Occasionally a leakage would take place in 

 the manner described above, but the fissures in these cases were gross 

 and visible, and their direction had no reference whatsoever to that of 

 the structure. Indeed, as the leakage always took place towards the 

 surface, and not into the depth of the ice, I am inclined 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 

 Couvercle ; and on different parts of the Glacier du Geant, and every- 

 where 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 surface for itself, is important, as 

 it shows that the absence of infiltration 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 ^ to |rds of ah inch. Filled with the 

 infusion and surrounded with ice, this cup remained for two hours 

 without showing a trace of infiltration along its structural planes. 



I can only conclude from these experiments, that the chief sub- 

 stance 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 



