GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 125 



dition for the crystallization of the water is supplied successively parallel to the 

 refrigerating surface, whatever may be its position or form, and the molecules 

 yield to the relatively more powerful forces which are operative in the planes of 

 the secondary axes. 



e. Glacial granules. Glacial ice, which has not been subjected to a melting 

 temperature, is firm, solid, and, apparently, homogeneous, except for air bubbles 

 and foreign matter that it may contain. It is brittle, breaks without cleavage, 

 and in quantity, when pure, has a rich blue color by transmitted light. Sub- 

 jected slowly to a melting temperature there is developed a system of delicate 

 capillary tubes, which form a network throughout all the ice affected, and ex- 

 tend into the body of the glacier a number of feet. These tubes outline the 

 granules, more or less perfectly, of which the entire glacier is composed. These 

 granules are irregular polyhedrons, of variable size, with curved faces which 

 interlock with one another. Ordinarily there are no spaces between them that 

 can be recognized and there is no cementing material to bind the granules 

 together. They are observed to increase in size from the neVe" to the nose in any 

 particular glacier and there can be no doubt but that the granules formerly in 

 the ne"ve are directly related to those seen in the lower part of the glacier. In the 

 Canadian glaciers studied the largest granules were seen in the basal layers 

 about the nose; the Asulkan, the smallest of the glaciers, having the smallest 

 average granules, and the Yoho, the largest glacier, having the largest average 

 granules. When subjected to considerable melting the capillary tubes become 

 "irregular and very thin spaces open between the faces of adjoining granules, 

 allowing the granules eventually to fall apart, or to be easily pulled apart. 



Each granule is an incomplete ice crystal, incomplete because its development 

 has been interfered with by the neighboring crystals. Belonging to the hex- 

 agonal system of minerals, it has a single main axis, which is also its principal 

 optic axis. In common with all known ice crystals it appears to be made up of a 

 bundle of very thin plates, placed with their flat faces together, the axis standing 

 at right angles to these plates. When the granules have melted apart the very 

 delicate edges of these plates, or more probably sets of these plates, may often 

 be recognized extending as delicate parallel lines about the granule and thus indi- 

 cating the positions of the planes of the secondary axes. These lines are known as 

 " Forel's stripes." They are referred to by Mxigge as the "Translations Streifung," 

 and were regarded by him as due to the partial shearing of the basal planes over 

 one another. They are found, however, in newly forming crystals of ordinary 

 lake or pond ice which have not been subjected to any shearing stress. Within 

 the body of the granule there are seen, at times, circular disks of excessive 

 thinness, with their flat faces perfectly parallel and all at right angles to the optic 

 axis. They are of silvery whiteness and appear like "flattened air bubbles," 

 as they were originally described by Agassiz (plate vi of Atlas, figure 10). These 

 are "Tyndall's melting figures" and are cavities, "vacuous space," con- 

 taining nothing more than water vapor, resulting from the internal contrac- 

 tion of the water, as it changes from its solid to its denser liquid condition. 



