494. Mr. J. Ball on the Structure of Glaciers. 
first spoken of, no objection could be made to it; but if I am 
not under a grievous misconception in believing that the blue 
veins may usually be traced for a distance of many yards, and 
almost constantly for several feet, I may be permitted to appeal 
to the subsequent and wider experience of the authors of this new 
term against the use of it as generally descriptive of the pheno- 
menon to which they seem disposed to apply it. 
3. Dirt-Bands of Professor Forbes.—The appearance first de- 
scribed by Professor Forbes under this name has been confounded 
by subsequent writers with a totally different phenomenon to 
which I shall presently refer, and this makes it necessary to 
distinguish it by his name. It is generally to be recognized 
only from a distance, when a large extent of glacier is seen . 
under such conditions of light and shade as enable the eye to 
seize slight variations of hue; but when once seen it is impossible 
not to come to the conclusion that it is an indication of the 
working of some mechanical law, which has operated over the 
whole mass of the glacier. 
Professor Tyndall has illustrated, by ingenious experiments, 
the conclusion drawn in the paper so often cited, which refers 
the production of these broad bands, or zones, of ice alternately 
white or discoloured, to the effect of steep falls or ice-cataracts 
separating into distinct portions the dirt which had before been 
evenly distributed over the surface. The form subsequently 
assumed is easily shown by him to be a consequence of the law 
of glacier motion. Though I believe that the effect of ice- 
cataracts in breaking up the surface of the glacier has been 
somewhat overrated*, I am inclined to think that the appear- 
ances produced in Professor Tyndall’s models are really traceable 
on some glaciers, but that they are different, both in form and 
origin, from those described by Professor Forbes. The latter 
were most accurately described in the third and fifth of his pub- 
lished letters. They traverse the whole breadth of a glacier 
stream, and are arranged at tolerably uniform intervals of several 
hundred feet, the breadth of the dirt-bands and of the intervening 
whiter spaces being approximately equal. I am not aware 
whether accurate measures have verified Professor Forbes’s sur- 
mise, that they correspond to slight undulations in the surface of 
the glacier, a fact which, if well established, would go far to 
prove his conclusion, which has always seemed to me probable, 
that they are the indications of an undulation in the flow of the 
glacier, depending on the different rates of progress in summer 
and winter. Fresh snow interfered with my observations on two 
or three occasions during the last season, but I have an impres- 
sion that during former visits to the Alps I have noticed an ar- 
* They certainly do not seem to affect the continuity of moraines, 
