But the rapid advance of spring added a new source of surprise to the lad. From the stillness and silence of the days when they first made camp at the head of the pass, the air became filled with the myriad voices of life, and the primal solitude became vibrant with tiny songsters. The golden sparrow was there with his piercing plaint, made musical by distance, and the trilling warble of Townsend's fox sparrow, and the varied strain of the hermit thrush, seemed quite homelike. Before the snow was gone the rosy finch was to be seen, his quick flight giving a gay spot of color to the landscape, and that the more utilitarian side might not be omitted, the snowy ptarmigan formed a welcome addition to the larder of the camp. Quite a torrent was beginning to flow over the ice in the Jack River, and on the morning of May 16th, when Roger had gone out with Gersup the topographer, to map out with greater detail a little piece of country which had been passed by on a previous expedition, he saw that the center of the ice in the river was bulging up like a hog-backed bridge. "What makes it bulge that way?" asked the boy. "You should have been able to figure that out," was the response. "When the ice thaws it increases the volume of water under the ice. The edges are frozen solid to the land, the middle is more or less elastic, and so of course the sides stay solid and the middle heaves up. In warmer climates, it is the edge that thaws out first, but up here the rivers, strictly speaking, do not thaw free, the ice is forced from them by the spring floods. It is strictly a break-up rather than a thaw, although it gets warm thereafter very rapidly." "It certainly does," replied Roger, mopping his forehead. "It's hot enough now, and this is only the middle of May; while two weeks ago it was snowing like Billy O." On May 18th the ice broke, moved down about a mile and jammed, and a few hours later broke again, finally clearing from the upper reaches of the Jack River on the 19th. Rivers was delighted at the opportunity to get out so soon, as he had feared it might be as late as the first week in June before he could get away. "I think, Doughty," he said to Roger, "that we can count easily now on accomplishing what we set out to do, and probably get into the Arctic Ocean in good time for an early return." "That is, barring accidents," put in the topographer. "We will make up our minds not to have any," replied the chief of the party.