ON THE STRUCTURE AND MOTION OF GLACIERS 21 
which ought to be expected. It may be urged, that the structure is 
here developed, because of the sliding motion produced by the swifter 
flow of one of the glaciers; but some of the experiments with the 
model were so arranged, that both of the branch streams flowed with 
the same velocity ; the distortions, however, were such as are shown in 
the figure. The case is precisely the same in nature. On reference 
to the map of M. Agassiz, we find a straight line set out across the 
Unter Aar glacier bent in three successive years into a curve; but on 
the central moraine, which marks the common limit of the constituent 
streams, we find no breach in the continuity of the curve, which must 
be the case if one glacier slid past the other. 
§ 6. On the Dirt-Bands” of Glaciers 
Wherever the veined structure of a glacier is highly developed, the 
surface of the ice, owing to the action of the weather, is grooved in 
accordance with the lamination underneath. These grooves are some- 
times as fine as if drawn by a pencil, and bear in many instances 
a striking resemblance to those produced by the passage of a rake 
over a gravelled surface. In the furrows of the ice the smaller 
particles of dirt principally rest, and the direction of the furrows, 
which always corresponds with that of the blue veins, is thus rendered 
so manifest, that a practised observer can at any moment pronounce 
upon the direction of the lamination from the mere inspection of the 
surface of a glacier. But besides these narrow grooves, larger patches 
of discoloration are sometimes observed, which take the form of 
curves sufficient in width to cover hundreds or thousands of the 
smaller ones. To an eye placed at a sufficient height above a glacier 
on which they exist, their general arrangement and direction are 
distinctly visible. To these Professor Forbes has given the name of 
“Dirt-Bands,” and the discovery of them, leading as it did to his 
theories of glacial motion, and of the veined structure of glacial ice, is 
to be regarded as one of the most important of his observations. 
On the evening of the 24th of July he walked up the hill of 
Charmoz to a height of about 1000 feet above the level of the glacier, 
and, favoured by the peculiar light of the hour, observed “a series of 
nearly hyperbolic brownish bands on the glacier, the curves pointing 
downwards and the two branches mingling indiscriminately with the 
moraines.” The cause of these bands was the next point to be 
considered, and his examination of them satisfied him “that the 
particles of earth and sand and disintegrated rock, which the winds 
and avalanches and water-runs spread over the entire breadth of the 
