384 Professors Tyndall and Huxley on the Structure 



streams met, and by their mutual push actually squeezed the 

 circles into lines. Along this central portion in the glacier itself 

 the great medial moraine stands, and under it and beside it, as 

 already stated, the lamination is most strikingly developed ; the 

 blue veins being pai-allel to the axis of the glacier, or, in other 

 words, coinciding with the direction of the central moraine. 

 Midway between the moraine and the sides of the glacier the 

 structure is very imperfectly developed, and the deportment of 

 our model, which shows that the circles here scarcely change 

 their form, tells us that this is the result which ought to be ex- 

 pected. 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 flgure. The case is precisely the same in nature. 

 On reference to the map of M. Agazziz, 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 sometimes 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 fui-rows 

 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 j" 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 obser- 

 vations. 



On the evening of the 24th of July he walked up the hill of 



