APPENDIX, No. I. 



AN ATTEMPT TO ILLUSTRATE THE OEIGIN OF " DIRT- 

 BANDS " IN GLACIERS. By A. MILWARD, Esq.* 



" It will be remembered that Professor Forbes, in his interesting 

 work on the Glaciers of the Alps, describes the " dirt-bands " to be 

 nothing more than curved bands of porous ice, the surface of which 

 affords a readier receptacle to the drift of the glacier. He found the 

 dirt to be superficial, and merely an indication that the glacier is made 

 up of two kinds of ice, the one more porous than the other ; so that 

 the dirt lodges in the one more readily than in the other. The 

 question to be determined is, how we are to account for the existence 

 of the different kinds of ice thus regularly alternating. The dirt 

 being merely accidental to this subject of inquiry, it will be better to 

 speak of the dirt-bands, and the intervals between them, as the alter- 

 nating bands of porous and compact ice.f 



" The dirt-bands are found to follow the direction of the hyperbolic 

 curves marked out by the outcrop of the structural planes, known by 

 the name of the ribbon- structure. The ice forming the dirt- bands is 

 made up of that structure, in the same way as the other ice ; and 

 depends, of course, upon those laws in obedience to which the ribbon- 

 structure originates. For this reason, the curve of the bands is, like 

 that of the structure, found to be elongated low down the glacier, and 

 compressed as we approach its source. We have thus only to account for 

 the existence of bands of different kinds of ice, the form of curve of those 

 bands being explained in the same way as that of the ribbon-structure. 



" It may be observed, that the superior distinctness of the dirt- 

 bands, as we proceed lower down the glacier, is not necessarily an 

 evidence that the bands of porous and compact ice are there more 

 decidedly developed, but only that they are more distinctly apparent. 

 And this, I imagine, arises from the fact, that the lower ice has been 

 washed over for years ; and thus the pores have become more dis- 

 coloured by the deposit of drift than the pores of the corresponding 

 porous ice above. 



* [From the Edinburgh New Philosophical Journal. Jan. 1849.] 

 f These terms are, of course, only relative. 



