Page:Popular Science Monthly Volume 5.djvu/425

Rh in the one case liquefaction is produced by pressure, in the other by solar heat. Gradually the under-layers become incipient glacier-ice.

Movement of the mass originates in its gravity, and the direction must be down the slope on which it lies. Many streams in this way blend into immense rivers of ice, often several hundred feet in depth. The snow when first consolidated is filled with air-bubbles, and is white and opaque. Its whiteness disappears by expulsion of the air-bubbles



from pressure as the glacier moves down the valley. At its termination the ice is transparent, and its exquisite tints of blue indicate the extreme minuteness of the reflecting surfaces which linger in it.

The physical properties of ice by which it flows need not be