Mr. J. Ball on the Structure of Glaciers, 503 
that the proofs given by Professor Forbes were already sufficient 
and complete. 
I cannot, indeed, conceive any explanation of the facts known 
as to glacier movement which can consist with the entire absence 
of minute cracks (whether capillary or not) penetrating that part 
of the ice which is in course of gradual motion. True it is 
that under pressure these cracks will be closed up again, and 
therefore those parts of the ice not actually undergoing the pro- 
cess which causes the motion may well be free from fissures. 
Pending further observations, which I trust Mr. Huxley may 
be induced to make, I shall venture to adhere to the belief that 
the irregular network of fissures which pervades the surface of 
the ice when exposed to air and warmth, represents a structure 
already existing in the ice before it came to the surface; and 
that former fissures, though possibly closed so as to show no 
trace under the microscope, may yet be surfaces of easy melting, 
which on the first application of heat are recalled into existence. 
Air-bubbles in the ice.—In common, as I suppose, with many 
readers of Mr. Huxley’s very interesting observations, the absence 
of more precise details as to the circumstances under which they 
were made is to me a matter of regret. It is allowable to sup- 
pose that his visits to the glaciers were generally made in the 
middle of the day, and during fine weather, and in that case I 
feel some doubt whether the ice within 8 or 10 inches of the 
surface is in the same conditions of temperature as that of the 
interior of the glacier. I would submit that the hour of the 
day, the state of the weather, and of the thermometer, at the 
time and previous to the observations, and also a statement of 
the precautions taken to prevent the pieces of ice taken out from 
undergoing changes by exposure to sun and air, would be a 
valuable addition to the interesting particulars for which we are 
indebted to Mr. Huxley. Such details seem necessary for satis- 
factory proof of Mr. Huxley’s conclusion, that the entire ice of 
the glacier is pervaded by cells containing both water and air, a 
fact which seems, however, no way inconsistent with rational 
views of the conditions of temperature in the interior of glaciers. 
Dirt-streaks.—1 believe that there is no material inconsistency 
between what I have said as to the dirt-bands of M. Agassiz, and 
Mr. Huxley’s observations on the same phenomenon, for which 
I gladly adopt the name “ dirt-streaks.” I would observe, how- 
ever, that the illustration given at pl. 5 of the ‘Travels in the 
Alps,’ to which Mr. Huxley refers his readers, is far less accurate 
than most of those contained in the same work. A glance at 
the plate is sufficient to show that the lines represented on the 
glacier cannot possibly represent the same thing in the fore- 
ground, sixty or eighty yards from the eye, and in the middle 
