The tusks of the mastodon, moving together, dipped gently and easily downward and both boys shot off them into space. It was a matter of twenty feet to the soft snow, and they plunged into it out of sight. Behind them came the great tusks, hundreds of pounds of weathered ivory, plunging through the snow nearer the base of the cliff. They missed the two by a little, but they missed them. Harry felt himself smothered in a whirl of snow, then falling again for a short distance, and finally brought up on a soft turf, where he lay for a moment half dazed by the thud with which he struck. Then he scrambled to his feet and looked around. He was in a low-roofed, wide cavern, dusky with a greenish pale twilight. Joe was sitting up on the ground by his side, rubbing his elbow and leg alternately and looking foolish, as no doubt he felt. “Where are we, anyway?” asked Joe, and the query was pertinent if the answer which he got was not. “Riding the elephant,” replied Harry, with a rueful grin. Over their heads, ten feet away in the snow roof through which they had come, were four holes which let in the nebulous twilight by which they saw. They and the mastodon tusks had come that way. To get back was another matter. They looked about with much curiosity not untempered with dismay. They were beneath the crust of an enormous drift that the winter storms had whirled over the mastodon cliff. Under their feet was a mixture of mud and bones from the cliff, carpeted with grass and moss. Around them grew willows. The slender top branches of these had been caught by the first damp snow of early autumn and bent beneath it till they twined, holding the bulk of it up. This had frozen there and the succeeding snows had piled above it, leaving the place free, an ideal natural cold frame for the shrubs and grass of the bottom land. These appreciated the shelter, and feeling the thrill of spring in their dark world, were already putting forth young green leaves. Up and down stream the cavern extended indefinitely. On one side it ended abruptly against the cliff, on the other it tapered down to the river ice, already worn thin on its edge and beginning to thaw.