Over some portions of the surface of the glacier there is a miniature river system, consisting of a main stream, with numerous tributaries, but all flowing in channels of deep blue ice. Before reaching the front of the glacier, however, each one of these plunges down into a crevasse, or _moulin_, to swell the larger current, which may be heard rushing along in an impetuous course hundreds of feet beneath, and far out of sight. The portion of the glacier in which there is the most rapid motion is characterised by innumerable crags and domes and pinnacles of ice, projecting above the general level, whose bases are separated by fissures, extending in many cases more than a hundred feet below the general level. These irregularities result from the combined effect of the differential motion (as illustrated in the diagram on page 4), and the influence of sunshine and warm air in irregularly melting the unprotected masses. The description given in our introductory chapter of medial moraines and ice-pillars is amply illustrated everywhere upon the surface of the Muir Glacier. I measured one block of stone which was twenty feet square and about the same height, standing on a pedestal of ice three or four feet high. The mountains forming the periphery of this amphitheatre rise to a height of several thousand feet; Mount Fairweather, upon the northwest, from whose flanks probably a portion of the ice comes, being, in fact, more than fifteen thousand feet high. The mouth of the amphitheatre is three miles wide, in a line extending from shoulder to shoulder of the low mountains which guard it. The actual water-front where the ice meets tide-water is one mile and a half.[AI] Here the depth of the inlet is so great that the front of the ice breaks off in icebergs of large size, which float away to be dissolved at their leisure. At the water's edge the ice presents a perpendicular front of from two hundred and fifty to four hundred feet in height, and the depth of the water in the middle of the inlet immediately in front of the ice is upwards of seven hundred feet; thus giving a total height to the precipitous front of a thousand feet. [Footnote AI: These are the measurements of Professor Reid. In my former volume I have given the dimensions as somewhat smaller.]