Page:Popular Science Monthly Volume 5.djvu/427

Rh is shown the source of the Arveiron and the sublime view of the foot of the glacier whence it issues. Of this M. Rendu says, "It is a vast portico more than a hundred feet high, let into an immense façade, and surmounted by lofty pyramids of ice. Nothing is more astonishing than this work of the elements, of which Nature alone has conceived the plan, and achieved the construction."



Ice presents, under pressure, many phenomena of great interest other than those mentioned, and to which we can only refer. The prismatic or crystalline form, so beautifully developed in lake-ice, is more or less destroyed in glaciers by the unceasing fracture and regelation which takes place from pressure, and the mass assumes a granular structure. The same phenomenon occurs with ice in a mould. In glaciers are veins which reflect a deeper tint of blue, indicating where from local causes greater or more persistent pressure has cleared it of bubbles of air. Glacier, and probably other ice, under similar conditions of pressure, becomes laminated, or develops planes of cleavage, resembling those of slate-rock in the quarry; and this structure is shown in the decay of the ice, as its prismatic structure is shown in the decay of that on lakes and rivers.

By the physical properties we have noticed, ice becomes a dynamic agent of tremendous power. The play of forces and plasticity of structure which make it a toy in the laboratory, have changed the aspects of Nature, and modified the surface, as they have the distribution of life, upon a large portion of the globe. Rocks are broken by its expansive energy, but they are also crushed by its weight, and ground to dust in its irresistible motion. A sheet of ice a mile in