Prof. Huxley on the Structure of Glacier Ice. 259 
surface of the ice is more or less dirty, not half-a-dozen square 
inches being without its little grains of sand and minute gravel, 
brought down, as I imagine, by the water which continually 
trickles from above; but the greater part of this impurity is 
invisible from a small distance, unless where it is specially accu- 
mulated. 
Such accumulation takes place in two localities; in the first 
place, on the little shelves afforded by the upper and more 
southerly aspects of the “ ripples” above referred to. Here the 
dirt accumulates quite independently of the structure, and as a 
consequence either of the form or of the aspect of the part on 
which it lodges. 
From a little distance these aggregations appear as spots and 
patches, but further off they cease to be visible, and the glacier 
between the horizontal streaks appears white. 
These streaks mark the second locality in which the dirt aggre- 
gates. Now whenever I carefully examined the surface of the 
glacier at these points, I found it to be weathered into large gra- 
nules, separated by coarse fissures which extended for a consider- 
able depth into the substance of the glacier; while the parts 
intermediate between the streaks, and which appear white from 
a distance, presented very much smaller granules, with fissures 
proportionately finer, and extending inwards for but a very small 
distance. In short, where the dark streaks existed, the ice was 
deeply weathered and coarsely granular, affording lodgment for 
dirt to a depth of two or three inches; while the intermediate 
substance had undergone only superficial weathering, and its 
finely granular structure afforded but little facility for the intru- 
sion of foreign matters. 
The “ dirt-streaks,” then, are due to the unequal weathering 
of the ice ; but why does the ice weather unequally? On seek- 
ing for an answer to this question, I found that every dirt-streak 
corresponded either with a very large blue vein, or with a closer 
aggregation than usual of smaller blue veins, while the interme- 
diate substance contained a preponderance of the smallest blue 
veins ; so that the coarse granules were the result of the weather- 
ing of parts of the glacier, composed either exclusively, or for the 
most part, of blue ice, while those in which the proportion of 
white ice was larger, weathered less deeply and into finer granules. 
The markings of La Brenva, then, are neither ordinary dirt- 
bands, nor direct expressions of structure, nor direct evidence of 
stratification, but they are produced by the more ready lodgment 
of dirt in some parts of the superficial layer of the glacier than in 
others, in consequence of the more coarse and deep weathering of 
these parts; which, again, is the result of the predominance of 
blue ice over white in these localities. 
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