————— ee 
Mr. J. Balk on the Structure of Glaciers. 487 
Again, if the veined structure be produced by pressure alone, 
how does it happen that the part of the mass which must be 
most amenable to the action of external force, I mean the white 
ice filled with air-bubbles, does not show in the form of the 
cavities which pervade its whole substance, a record of the 
process by which it is assumed that the internal condition of the 
ice has been so profoundly modified? But as Agassiz has pointed 
out, and I have frequently verified his observations on this point, 
though the air-cavities often show traces of compression reducing 
them to the form of flattened lenses, the directions in which they 
are flattened are most various, and show no constant relation to 
the planes of the veined structure. Here then we have direct 
evidence that separate portions of the ice have been acted on by 
pressure sufficient in amount to modify their internal arrange- 
ment, but that these pressures have not acted in the same, or 
nearly the same direction. But surely this is not consistent 
with the supposed action of force on a great scale upon an 
enormous mass of ice—able to destroy first the primitive struc- 
ture, and then to impress upon it anew and different one. Such 
a force must have entered into composition with any local pres- 
sures which can be supposed to have existed in the interior of 
the mass, and the resultants would have approximated closely to 
the general direction of its action. 
A further difficulty which I think deserves some attention is 
this, that the pressure hypothesis gives no explanation whatever 
of the irregularity in the appearance and the thickness of the 
ves. Even if we are to admit that such pressure as exists in 
glaciers is competent to impress upon masses of névé, sometimes 
many square miles in extent, a veined structure consisting of 
alternate bands of nearly compact ice and ice filled with air- 
bubbles—a conclusion, let me say, which is not absolutely in- 
consistent with what we know of the action of pressure on seem- 
ingly homogeneous masses of matter, but which has not yet 
been shown to be possible, either by experiment or by any close 
accordance with observed facts,—is there anything that would 
lead us to expect that the veins should be otherwise than mode- 
rately uniform in structure? Every one, however, who has 
looked carefully at the veined structure knows that this is not 
the fact : some of the blue veins are very thin and faintly marked, 
others on the contrary much thicker, more compact, and there- 
fore more transparent than those on either side of them. But 
there is a further fact to which I wish to call particular atten- 
tion, viz. the recurrence at regular intervals of these thicker and 
more transparent blue veins. I do not feel justified in asserting 
that this is constantly to be seen, but I have reason to think it 
very frequent. My attention was first called to it on a small 
