Land-ice, of which both the ice-fields of the Arctics and glaciers consist, is produced by the slow and gradual transformation of snow into ice; and though the ice thus formed may eventually be as clear and transparent as the purest pond- or river-ice, its structure is nevertheless entirely distinct.  We may trace these different processes during any moderately cold winter in the ponds and snow-meadows immediately about us.  We need not join an Arctic exploring expedition, nor even undertake a more tempting trip to the Alps, in order to investigate these phenomena for ourselves, if we have any curiosity to do so.  The first warm day after a thick fall of light, dry snow, such as occurs in the coldest of our winter weather, is sufficient to melt its surface.  As this snow is porous, the water readily penetrates it, having also a tendency to sink by its own weight, so that the whole mass becomes more or less filled with moisture in the course of the day.  Daring the lower temperature of the night, however, the water is frozen again, and the snow is now filled with new ice-particles.  Let this process be continued long enough, and the mass of snow is changed to a kind of ice-gravel, or, if the grains adhere together, to something like what we call pudding-stone, allowing, of course, for the difference of material; the snow, which has been rendered cohesive by the process of partial melting and regelation, holding the ice-globules together, just as the loose materials of the pudding-stone are held together by the cement which unites them.

Within this mass, air is intercepted and held inclosed between the particles of ice.  The process by which snow-flakes or snow-crystals are transformed into grains of ice, more or less compact, is easily understood.  It is the result of a partial thawing, under a temperature maintained very nearly at thirty-two degrees, falling sometimes a little below, and then rising a little above the freezing-point, and thus producing constant alternations of freezing and thawing in the same mass of snow.  This process amounts to a kind of kneading of the snow, and when combined with the cohesion among the particles more closely held together in one snow-flake, it produces granular ice.  Of course, the change takes place gradually, and is unequal in its progress at different depths in the same bed of recently fallen snow.  It depends greatly on the amount of moisture infiltrating the mass, whether derived from the melting of its own surface, or from the accumulation of dew or the falling of rain or mist upon it.  The amount of water retained within the mass will also be greatly affected by the bottom on which it rests and by the state of the atmosphere.  Under a certain temperature, the snow may only be glazed at the surface by the formation of a thin, icy crust, an outer membrane, as it were, protecting the mass below from a deeper transformation into ice; or it may be rapidly soaked throughout its whole bulk, the snow being thus changed into a kind of soft pulp,