Origin of^^ Dirt-bands''' in Glaciers. 137 



becomes precisely analogous to that of the mud-slide : The 

 lower extremity of each ridge will be more broken up than 

 the other parts, just as we see the ridges and lower extre- 

 mities of the several mud-streams to be broken up and more 

 porous. From these high and broken parts, the water will 

 drain and saturate, to a greater extent, the surfaces below. 

 The ice thus saturated will become, by the action of frost, 

 more compact than the rougher ridges. This fact, that sa- 

 turated ice produces the most compact glacier, appears to 

 have been already assumed (either from experience or other- 

 wise), in the explanation which is given of the formation of 

 the transparent blue bands of the ribbon-structure. An ob- 

 jection to which this explanation is open, will be found in 

 the very different width of the porous and compact bands on 

 the glacier ; whereas in the mud-slide they are nearly equal. 



In any case, however, it seems that there is sufficient 

 ground why we should look for such ridges or waves ; but at 

 the same time, it is quite possible that they may be proper 

 to a peculiar condition of viscous matter, to which class the 

 glacier does not belong, or it may be that special resistance 

 and obstruction prevents the development of such peculia- 

 rity in the case of glaciers. If this be so, the direct ana- 

 logy between the glacier and the mud-slide in this respect 

 vanishes. 



3. From the manner in which the second mud-slide ex- 

 plains the first, — depending, it will be remembered, on the 

 drainage of water altering the character of the mud, — another 

 deduction may, I think, be drawn as applicable to glaciers, 

 although not exactly in the same way. The result to which 

 I am about to draw your attention must., I think, have a real 

 existence. It is a result to whicll the dirt-bands may be the 

 indication, or it may have passed altogether unobserved. 



We find at the heads of most glaciers, where the neve is 

 passing into ice, and the body assuming its normal form and 

 construction, that there are steeper elevations, from which 

 the neve descends, and frequently ice-cascades. The glacier 

 is in these parts, on account of the abrupt descent, very 

 much broken up, and often impassable. Now it appears evi- 

 dent, that, at the foot of these slopes, the water which has 



