Ice of this kind, resulting from the direct congelation of water, is easily recognized under all circumstances by its regular stratification, the alternate beds varying in thickness according to the intensity of the cold, and its continuance below the freezing-point during a longer or shorter period. Singly, these layers consist of irregular crystals confusedly blended together, as in large masses of crystalline rocks in which a crystalline structure prevails, though regular crystals occur but rarely. The appearance of stratification is the result of the circumstances under which the water congeals. The temperature varies much more rapidly in the atmosphere around the earth than in the waters upon its surface. When the atmosphere above any sheet of water sinks below the freezing-point, there stretches over its surface a stratum of cold air, determining by its intensity and duration the formation of the first stratum of ice. According to the alternations of temperature, this process goes on with varying activity until the sheet of ice is so thick that it becomes itself a shelter to the water below, and protects it, to a certain degree, from the cold without. Thus a given thickness of ice may cause a suspension of the freezing process, and the first ice-stratum may even be partially thawed before the cold is renewed with such intensity as to continue the thickening of the ice-sheet by the addition of fresh layers. The strata or beds of ice increase gradually in this manner, their separation being rendered still more distinct by the accumulation of air-bubbles, which, during a hot and clear day, may rise from a muddy bottom in great numbers. In consequence of these occasional collections of air-bubbles, the layers differ, not only in density and closeness, but also in color, the more compact strata being blue and transparent, while those containing a greater quantity of air-bubbles are opaque and whitish, like water beaten to froth.