Although I had, in my overland journey from Van Rensselaer Harbor with Mr. Wilson, in 1853, reached the face of the _mer de glace_, where it rested behind the lofty chain of hills which runs parallel with the axis of the continent, yet this was the first time that I had actually been upon it; and its vastness did not on the former occasion impress me as now. Even the description of the great Humboldt Glacier which I had from Mr. Bonsall, and the knowledge that I had acquired of the immense glacier discharges of the region further south, failed to inspire me with a full comprehension of the immensity of ice which lies in the valleys and upon the sides of the Greenland mountains. Greenland may indeed be regarded as a vast reservoir of ice. Upon the slopes of its lofty hills the downy snow-flake has become the hardened crystal; and, increasing little by little from year to year and from century to century, a broad cloak of frozen vapor has at length completely overspread the land, and along its wide border there pour a thousand crystal streams into the sea. [Sidenote: THE GLACIER SYSTEM.] The manner of this glacier growth, beginning in some remote epoch, when Greenland, nursed in warmth and sunshine, was clothed with vegetation, is a subject of much interest to the student of physical geography. The explanation of the phenomena is, however, greatly simplified by the knowledge which various explorers have contributed from the Alps,--a quarter having all the value of the Greenland mountains, as illustrating the laws which govern the formation and movements of mountain ice, and which possesses the important advantage of greater accessibility. It would be foreign to the scope and design of this book to enter into any general discussion of the various theories which have been put forth in explanation of the sublime phenomena, which, as witnessed in the Alpine regions, have furnished a fruitful source of widely different conclusions. It was, however, easy to perceive in the grand old bed of ice over which I had traveled, those same physical markings which had arrested the attention of Agassiz and Forbes and Tyndall, and other less illustrious explorers of Alpine glaciers; and it was a satisfaction to have confirmed by actual experiment in the field the reflections of the study. The subject had long been to me one of great interest; and I was much gratified to be able to make a comparison between the Alpine and Greenland ice. It was not difficult to read in the immense deposit over which I had walked whence came the suggestion of _dilatation_ to Scheuchzer, or of _sliding_ to De Saussure; or, in the steady progress of knowledge and discovery, the principles of action that are illustrated by the terms _vitrious_ and _viscous_ and _differential motion_, as applied to the Alpine ice by eminent explorers of later date. [Sidenote: GLACIERS.]