crystallization in freezing. The general law of structure in all masses slowly crystallizing from a state of fusion, is the production of a prismatic structure, perpendicular to the cooling surfaces. Basalt assumes its polygonal figures in obedience to the same law, and the structure of ice is quite in accordance with it." Starch is an example of this structure. In an ingot of tin it is concealed, but under a gentle heat the more fluid parts melt out, and leave a skeleton of crystalline prisms. When water freezes it expands about one-eleventh' of its volume. "The first result of the fusion of a part of it is to dissect out the prismatic masses, leaving them standing isolated by reason of their being on a larger scale than the fluid volume from which they were formed. In this process the air-bubbles no doubt materially assist by opening channels of escape for the ice water."

The writers just quoted suppose the prisms of ice to be formed during the freezing of the water. It is quite as probable that the prisms are formed during the thawing of the ice. The application of heat to vitreous substances and to some metals induces a crystalline character in bodies that did not previously possess that texture, and it is very possible that such is the case with ice. Mr. Graham's researches (Phil. Trans. 1861) seem to prove that ice may be both vitreous and crystalline.

The absence of horizontal layers noticed by the above writers is not a constant feature in ice.

Professor Silliman, writing from New Haven, in the severe winter of 1821, speaks of the regularity of the layers of ice produced by each night's congelation. The ice, it should be remarked, was that of a mill pond, and the water would be at rest in frosty weather; whereas in Lake Champlain the water was so agitated by the cold wind as to prevent convection, and to reduce the lake in its depths to near the freezing point before any ice was formed. In the mill pond, however, where all was at rest, the ice was formed in layers. "In masses 15 inches thick there were 21 distinct layers quite as distinct as those of agate or striped jasper, or of the annual rings produced by the growth of wood. These layers, near and at the top, were from 1 to 2 inches thick, and at the bottom, next the stream, from inch-giving an average of nearly

toof an of an inch."

CHAPTER II.

ON GLACIERS.

THE Contemplation of lofty mountains has always been found to excite a deep and peculiar influence on the human mind. There is something in their vast magnitude, in their cloudy height, and in their quiet majesty of repose, which overpowers and solemnizes the spectator, and inspires feelings of veneration and awe. Especially is this the case with snowy mountains: their shadowy outlines dimly reveal a region that seems to belong to the sky rather than to the earth, and which is pervaded by an impressive silence, and a mysterious grandeur of its own, beyond all power of description. If we penetrate this region, we find its white uniformity giving place to variety and beauty. The forms which at a distance looked like masses of cloud, assume solidity and are richly tinted by the sunbeams; rosy peaks and glistening pinnacles crown the heights, while many of the intervening valleys and plains present the wonderful sight of that frozen river, or icy stream, which we know as the GLACIER.

The glacier is formed on the heights of these vast mountains; for in its origin it is nothing more

than snow. The clouds which are formed above and around these heights discharge themselves in snow, just as the clouds which are formed in lower and more temperate regions discharge themselves in rain. Above a certain height the mountains are always covered with snow; below it the snow melts entirely away during summer. The limit of perpetual snow is called the snow-line, a term which has been thus clearly defined by one of our latest and most diligent observers of Alpine phenomena. "The term is usually understood to mean the line along which the quantity of snow which falls annually is melted, and no more. Below this line each year's snow is completely cleared away by the summer heat; above it a residual layer abides, which gradually augments in thickness from the snow-line upwards."*

When these residual layers have accumulated to a certain extent, occupying the slopes as well as the summits of lofty mountains, they slip and slide on each other, and at length roll down the steeps in avalanches, thus preventing the stupendous result which must otherwise take place; namely, the elevation of the snowy mountains, by successive accumulations, to an indefinite height. Cataracts of snow are thus, at intervals, thrown down the sides of the mountains, and the deep low sound of their fall breaks in solemnly on the universal silence; but these cataracts do not in themselves afford sufficient relief to the snowy

* Tyndall. "Glaciers of the Alps," p. 249.