this the air current rises and so crosses the mountain range, only to be met beyond by the still colder and loftier Rockies, where most of the balance of the moisture is lost. Herein we see why the western slopes of these mountains have a much heavier rain and snowfall than the eastern slopes and why it is that the great plains stretching from the foothills to the centre of the continent are comparatively so dry.
In the lower levels this precipitation is in the form of rain, during most of the year at least. But when we reach the elevation of the higher mountains it is almost entirely fine granular snow, even in midsummer. On bright days part of this will evaporate, but the greater portion keeps on accumulating, until as the result of the pressure of the superimposed weight of new snowfalls, it gradually becomes compacted into hard solid ice. This ice is like that which forms in our rivers and lakes, except that its internal crystalline structure is different, owing to the different way in which it was formed. Now, if this were the end, the situation in our regions of high mountains would be very different from what it is, for the snow and ice would keep on increasing indefinitely, as the amount of melting at such high levels must be quite small, and conditions analogous to those of the polar regions would ensue. But Nature comes to the rescue. With the increasing pressure caused by the weight of the ice, to which is added the attraction of gravitation, the ice starts to flow; very slowly, but none the less surely. It is hard for us to conceive of so brittle a substance as ice, as we know it, flowing. Yet it does; and doubtless the internal structure of the ice, above referred to, aids in this. But any ice under pressure is more or less plastic. The pressure exerted on these great bodies of ice by the weight above is tremendous, and their onward motion is resistless. Its effects are seen in the way in which ledges of the hardest rock are smoothed off, and often-
