Origin o/"" Dirt-bands^'' in Glaciers. 139 



It will result also from this theory, that the breadth of the 

 ** dirt-band" and interval, taken together, should equal the 

 annual advance of the glacier. And this appears, from obser- 

 vation, to be the actual case. We might also expect the re- 

 lative breadth of the dirt-bands and interval to approximate 

 towards the proportion of the winter and summer mean gla- 

 cial motion. There may, however, be causes, arising from 

 the positions and proportions of the lower and upper slopes 

 at the source of the glacier, which would disturb this pro- 

 portion between the two bands, even more than they would 

 alter the relation between the two bands taken together and 

 the annual glacial motion. 



It is with the greatest diffidence that the writer would ven- 

 ture to submit, that a prima facia case has been made out 

 for three subjects of inquiry. 



1st, Whether there are indications of the existence of wide 

 structural bands (of which the bands on the surface of the 

 mud-slide are the outcrop) in viscous fluids and glaciers. 



2t/, Whether there are any traces in the upper parts of 

 glaciers of ridges or waves answering to the ridges occurring 

 on the mud-slide. And, 



3(:?/y, Whether the saturation at the foot of the upper slope, 

 which must theoretically exist, is practically effective, so as 

 to cause the alternate bands of porous and compact ice, in 

 the manner which I have endeavoured to describe. 



Fifteenth Letter on Glaciers ; containing Observations on the 

 Analogies derived from Mud- Slides on a large Scale, and 

 from some processes in the Arts, in favour of the Viscous 

 Theory of Glaciers. Addressed to the Rev. Dr Whewell 

 by Professor Forbes. (With a Plate.) Communicated by 

 the Author. 



Edinburh, 2d December 1848. 



My dear Sir, — It is considerably more than a year since 

 you did me the favour to communicate to me the interesting 

 drawing and remarks by your friend Mr Milward, on a mud- 

 slide on a large scale, which had come under his observation 

 at Malta, and which led him to notice some interesting ana- 



