Heavy slides sometimes rush so swiftly down steep slopes that their momentum carries their entire mass destructively several hundred feet up the slope of the mountain opposite. Desiring fuller knowledge of the birth and behavior of avalanches, or snow-slides, I invaded the slide zone on snowshoes at the close of a winter which had the "deepest snow-fall on record." Several days were spent watching the snow-slide action in the San Juan Mountains. It was a wild, adventurous, dramatic experience, which closed with an avalanche that took me from the heights on a thrilling, spectacular coast down a steep mountain-side. [Illustration: LIZARD HEAD PEAK IN THE SAN JUAN MOUNTAINS] A thick, snowy, marble stratum overlay the slopes and summits. Appearing on the scene at the time when, on the steeps, spring was melting the icy cement that held winter's wind-piled snows, I saw many a snowy hill and embankment released. Some of these, as slides, made meteoric plunges from summit crags to gentler places far below. A snow-storm prevailed during my first night in the slide region, and this made a deposit of five or six inches of new snow on top of the old. On the steeper places this promptly slipped off in dry, small slides, but most of it was still in place when I started to climb higher. While I was tacking up a comparatively smooth slope, one of my snowshoes slipped, and, in scraping across the old, crusted snow, started a sheaf of the fluffy new snow to slipping. Hesitatingly at first, the new snow skinned off. Suddenly the fresh snow to right and left concluded to go along, and the full width of the slope below my level was moving and creaking; slowly the whole slid into swifter movement and the mass deepened with the advance. Now and then parts of the sliding snow slid forward over the slower-moving, crumpling, friction-resisted front and bottom. With advance it grew steadily deeper from constantly acquired material and from the influence of converging water-channels which it followed. A quarter of a mile from its birthplace it was about fifty feet deep and twice as wide, with a length of three hundred feet. Composed of new snow and coasting as swiftly as a gale, it trailed a white streamer of snow-dust behind. A steeper or a rougher channel added to the volume of snow-dust or increased the agitation of the pace-keeping pennant. The morning was clear, and, by watching the wigwagging snow flag, I followed easily the fortunes of the slide to the bottom of the slope. After a swift mile of shooting and plunging, the slide, greatly compressed, sprawled and spread out over a level glacier meadow, where its last remnant lingered for the warmth of July. Dismissing this slide, I watched along the range to the north and south, and from time to time saw the white scudding plumes of other slides, which, hidden in the cañons, were merrily coasting down from the steep-sloping crest.