WHAT IS A GLACIER? 7 



is brought out. The powder of almost any substance differs 

 in color from the compact mass. Glacial ice is compressed 

 snow, and originates wherever the snow-fall is largely in ex- 

 cess of the melting power of the sun and warm currents of air. 

 Any one can observe how much more compact old snow is 

 than new, and how, under pressure, the lower strata in a 

 snow-bank become in a single season almost like ice. Hence 

 it is easy to see what must be the result where the annual 

 snow-fall is never wholly melted away. In such regions the 

 ice would accumulate without limit, were it not for its semi- 

 fluid character, which permits it to flow off, in lines of least 

 resistance, to lower levels and toward warmer climes. 



In structure glacial ice is characterized by both veins and 

 fissures — two phenomena, which are produced by opposite 

 causes — the first by pressure, and the second by tension. 



Glacial ice ordinarily presents a veined structure. Instead 

 of being homogeneous, it consists of alternate bands of light- 

 colored and blue ice. ' These bands do not, however, lie in a 

 horizontal position, but are often vertical. Sometimes they 

 run parallel with the movement of the glacier, and sometimes 

 at right angles to the motion ; while, at other times, they are 

 arranged at an angle of forty-five degrees, pointing down the 

 line of motion. From close examination it appears that the 

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



For example, where two branches of a glacier join, and 

 press together from the sides, longitudinal veins are produced 

 below the point of junction. And again, where ice has de- 

 scended a declivity, and is advancing upon a less inclined 

 plane, the increased pressure necessary to push the mass along 

 produces bands at right angles to the line of motion ; thus 

 demonstrating the connection of veins with pressure. The 

 theory is, that the blue veins in the ice are those from which 

 pressure has expelled the particles of air, thus making it 

 more compact, and giving it its blue color. As already re- 

 marked, snow is white because of the abundant particles of 

 air inclosed within it. Under pressure it can be transformed 

 into blue ice, corresponding to the blue veins alluded to 

 4 



