GLACIERS OP THE CANADIAN ROCKIES AND SELKIRKS. I 19 



to them by Forbes, their discoverer, and is in more general use. The trans- 

 verse, parallel troughs, in which the dirt bands have their origin, arise from the 

 incomplete healing of .ransverse crevasses which occur at the crest of a steep 

 ice slope. The lips of a crevasse, exposed to intense solar action are rounded 

 more or less and when the crevasse closes there is left a trough which marks the 

 position of the original crevasse. Into this depression wind-blown dust collects 

 and is washed from the adjacent slopes. By absorbing heat this dust may em- 

 phasize the depression slightly and may render the ice somewhat spongy, as 

 pointed out by Tyndall. If the ice slope is too steep, a cascade results and the 

 ice is too much shattered to show the bands, or to allow them to form. If the 

 slope is steep, but regular, with much melting over the surface, the site of 

 the bands will be destroyed before any complete series can develop. Conditions 

 are most favorable for their production upon the face of a moderately steep slope, 

 which is immediately followed by a long stretch of gently inclined ice. They 

 sustain no necessary relation, whatever, to the dirt zones, being present when 

 the zones are absent. When both zones and bands are present they may be 

 comformable for a greater or less distance and may be difficult to distinguish 

 from one another. In the case of such a glacier as the parasitic Lefroy the zones 

 and dirt bands may be discordant and intersect at high angles. There is reason 

 for thinking that the dirt bands are produced annually, only the summer formed 

 crevasses furnishing the necessary troughs, while the few winter crevasses com- 

 pletely and perfectly heal in passing down the slope. If this proves to be the 

 case we have a means of determining the approximate yearly motion of the ice 

 along the slope and a clue to the extent of the longitudinal compression, or 

 extension, of the ice subsequently. 



Where the edges of the blue bands, embedded in the more porous whiter ice, 

 outcrop upon the surface, particularly along the margins of the glacier pressing 

 firmly against the valley wall, there is developed a further miniature banding. 

 The firmer blue ice melts less rapidly than the more vesicular layers and a series 

 of parallel ridges and troughs results, the course, distance, and average breadth 

 of which is determined by the ice structure itself. In the narrow troughs the fine 

 dirt collects and the ice is marked with a series of delicate parallel dirt streaks. 

 Tyndall compared them with the marks left in a gravel walk by a garden rake. 

 Drygalski describes them under the name of Schmutzbdnder, but this term must 

 be reserved for the true dirt bands of Forbes. Dirt stripes suggests their appear- 

 ance and will enable them to be distinguished from all the other dirt features. In 

 that they owe their existence to the actual structure of the ice they have some 

 relationship with the dirt zones, but in that the dirt of which they are composed 

 is purely superficial, they are more nearly related to the dirt bands. They are 

 to be seen at only a short distance, while the zones and bands are best brought 

 out from a distant, elevated view. 



6. Differential melting effects. Under favorable circumstances an interesting 

 series of stages may be passed through by dust wells; dirt, sand, and gravel 

 cones; boulder mounds; lakelets and morainic ridges. This was first worked out 



