they have often been compared. They conform more or less perfectly to the shape of the valley and irregularities of the bed, move more rapidly towards the centre and upper surface than toward the sides and bottom, flow more rapidly down steep slopes than gentle ones, and are more active during the day than at night and in summer than in winter. Where compelled to change their course too suddenly, or when subjected to a certain degree of tensional stress, great cracks are slowly opened at right angles to the direction of such stress. When one portion of the mass begins to lag it may be thrust forward bodily by great pressure from behind, compelled to mount reverse slopes, to scour the bed, detach rock fragments and transport whatever material finds lodgment within or upon the mass.
I. — Conditions Requisite for Glacial Formation.
In order that a certain region may support glaciers four conditions must be fulfilled, no two or three of which alone will suffice. (a) There must first be a degree of cold that will cause some of the precipitation to fall as snow or hail without which a glacier would be impossible, (b) The amount of such precipitation must be sufficiently great so that, in spite of the seasonal melting and evaporation, there will be a remnant to add to the accumulation of previous years. The entire snowfall of any 3'ear, or any short series of years, may be destroyed by melting, but, upon the whole, there must be more or less steady increase in the amount of congealed moisture, (c) There must be a collecting area, which from its shape or slope is capable of retaining the requisite amount of snow and ice. If the slope is too steep the snow will be avalanched from the area before the glacier has time to come into existence, (d) Finally, the local conditions must be of such a nature as to permit of the inauguration of a movement in which
