MR. MILWARD ON DIRT-BANDS IN GLACIERS. 265 



* dirt-bands/ and the narrow porous band to the ' dirt-band ' itself. 

 The one owes its formation to the summer, the other to the winter. 



" 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 observation, to be the actual 

 case. We might also expect the relative breadth of the dirt-bands 

 and interval to approximate towards the proportion of the winter and 

 summer mean glacial 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 proportion 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 venture 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 struc- 

 tural bands (of which the bands on the surface of the mud -slide are 

 the outcrop) in viscous fluids and glaciers. 



" 2d, 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<%, 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." 



Observations on the preceding communication, and especially on the 

 cause of the Annual Rings of Glaciers* By Professor FORBES. 



" Professor Forbes stated that Mr. Milward's shrewd suspicion of the 

 bands of ice of different consistence, being accompanied also by wrinkles 

 or elevations, had been discovered by himself some years before at the 

 very place and time pointed out as most likely ; and he shewed that, 

 while there is a tendency in a tenacious viscous fluid to produce wrinkles 

 under pressure, capable of effecting detrusion even where the supply of 

 the fluid is uniform, this quality is greatly increased when the supply of 

 the fluid is by fits, as it is in fact at the head of the glacier, where the 

 quasi-hydrostatic pressure from behind, combined with the frontal 

 resistance, produces a thickening, or convex lip or wrinkle. 



" He likewise mentioned the analogous instance of the production of 

 * Proceedings Royal Soc. Edin. 18th Dec. 1848. 



