Were the statements of some writers upon this subject well founded, or were the dirt-bands as drawn upon the map of Professor Forbes correctly shown, this explanation could not stand a moment. It has been urged that the dirt-bands cannot thus belong to a single tributary of the Mer de Glace; for if they did, they would be confined to that tributary upon the trunk-glacier; whereas the fact is that they extend quite across the trunk, and intersect the moraines which divide the Glacier du Geant from its fellow-tributaries. From my first acquaintance with the Mer de Glace I had reason to believe that this statement was incorrect; but last year I climbed a third time to the Cleft-station for the purpose of once more inspecting the bands from this fine position. I was accompanied by Dr. Frankland and Auguste Balmat, and I drew the attention of both particularly to this point. Neither of them could discern, nor could I, the slightest trace of a dirt-band crossing any one of the moraines. Upon the trunk-stream they were just as much confined to the Glacier du Geant as ever. If the bands even existed east of the moraines, they could not be seen, the dirt on this part of the glacier being sufficient to mask them. The following interesting fact may perhaps have contributed to the production of the error referred to. Opposite to Trelaporte the eastern arms of the dirt-bands run so obliquely into the moraine of La Noire that the latter appears to be a tangent to them. But this moraine runs along the Mer de Glace, not far from its centre, and consequently the point of contact of each dirt-band with the moraine moves more quickly than the point of contact of the western arm of the same band with the side of the valley. Hence there is a tendency to _straighten_ the bands; and at some distance down the glacier the effect of this is seen in the bands abutting against the moraine of La Noire at a larger angle than before. The branches thus abutting have, I believe, been ideally prolonged across the moraines. [Illustration: Fig. 38. Plan of Dirt-bands taken from Johnson's 'Physical Atlas.']