Although the ice of many glaciers, more especially thosewhich move over massive impervious rocks, is clean andalmost free from rock fragments, in many cases towards the bottomof the glacier, boulders, pebbles, and even masses of clay, mud, or sand abound. The rock-masses have evidently been derived from The floor over which the glaciers move, for the fragments become more numerous as we descend in the ice, the upper portion of the glacier being often quite free from foreign matter.