MAN AND THE GLACIAL PERIOD. 



Fig. 3 



Figs. 



bands are not arranged with any apparent uniformity in 

 the glacier, their explanation has given rise to much dis- 

 cussion. Sometimes the veins are horizontal, sometimes 

 vertical, and at other times at an angle with the line of 

 motion. On close investigation, however, it is found that 

 the veins are always at right angles to the line of greatest 

 pressure. This leads to the conclusion that pressure is 

 the cause of the banded structure. The blue strata in the 

 ice are those from which the particles of air have been 



expelled by pressure ; 

 the lighter portions are 

 those in which the par- 

 ticles are less thorough- 

 ly compacted. Snow is 

 but pulverized ice, and 

 differs in colour from 

 the compact mass for 

 the same reason that al- 

 most all rocks and min- 

 erals change their colour when ground into a powder. 



The fissures, which, when of large size, are called 

 crevasses, are formed in those portions of a glacier where, 

 from some cause, the ice is subjected to slight tension. This 

 occurs especially where, through irregularities in the bot- 

 tom, the slope of the descent is increased. The 

 ice, then, instead of moving in a continuous 

 stream at the top, cracks open 

 along the line of tension, and 

 wedge-shaped 

 fissures are 

 formed ex- 



Fig. 5.— c, c, show fissures and seracs where the glacier tending from 



the top down 

 to a greater 



or less distance, according to the degree of tension. Usu- 

 ally, however, the ice remains continuous in the lower 



Fig. 4. 



4. — Illustrate the formation of mar- 

 ginal fissures and veins. 



moves down the steeper portion of its incline ; s, ■<?, 

 show the vertical structure produced by pressure on 

 the gentler slopes. 



