have been polishing and fashioning the whole surface over which it extended.  As it now melts, it drops its various burdens on the ground; boulders are the mile-stones marking the different stages of its journey, the terminal and lateral moraines are the framework which it erected around itself as it moved forward, and which define its boundaries centuries after it has vanished, while the scratches and furrows it has left on the surface below show the direction of its motion.

All the materials which reach the bottom of the glacier, and are moving under its weight, so far as they are not firmly set in the ice must be pressed against one another, as well as against the rocky bottom, and will be rounded off, polished, and scratched, like the rock itself over which they pass.  The pebbles or stones set fast in the ice will be thus polished and scratched, however, only over the surface exposed; but, as they may sometimes move in their socket, like a loosely mounted stone, the different surfaces may in turn undergo this process, and in the end all the loose materials under a glacier become more or less polished, scratched, and grooved.  These marks exhibit also the peculiarity so characteristic of the grooves and scratches on the bed and walls of the valley:  they are rectilinear, trending in the direction in which the superincumbent mass advances, though, of course, owing to the changes in the position of the pebbles or boulders, they may cross each other in every direction on their surface.

As the larger materials are pressed onward with the finer ones, that is, with the sand, gravel, and mud accumulated at the bottom of the glacier, the component parts of this underlying bed of debris will be mixed together without any reference to their size or weight.  The softest mud and finest sand may be in immediate contact with the bottom of the valley, while larger rocks and pebbles may be held in the ice above; or their position may be reversed, and the coarser materials may rest below, while the finer ones are pressed between them or overlying them.  In short, the whole accumulation of loose debris under the glacier, resulting from the trituration of all kinds of angular fragments reaching the lower surface of the ice, presents a sort of paste in which coarser and lighter materials are impacted without reference to bulk or weight.  Those fragments which are most polished, rounded, grooved, or scratched, have travelled longest under the glacier, and are derived from the hardest rocks, which have resisted the general crushing and pounding for a longer time.  The masses of rock on the upper surface of the glacier, on the contrary, are carried along on its back without undergoing any such friction.  Lying side by side, or one above another, without being subject to pressure from the ice, they retain, both in the lateral and medial moraines, and even in the terminal moraines, their original size, their rough surfaces, and their