These waters repose in basins that have in miniature all the beauty of the Mammoth Cave. The basins and their rims are formed of minerals--mostly of silica--deposited by the water. The rims are fittingly beautiful; the lines of internal construction are harmonious. Many springs have built up their basins with precipitated minerals until they rest on mounds. All these are picturesque, and some are beautiful. Morning-Glory Spring is like a gigantic morning-glory set in the earth. The Firehole, with a black fissured bottom, has at times flamelike colors which create such an illusion that the fiery interior of the earth appears to be on exhibition. Prismatic Lake, a spring large enough to be called at least a lakelet or pond, is a combination of the artistic and the spectacular. It has built up for itself a rounded mound, and down the gently curving slopes flow its waters in thousands of interlacing rivulets. Over the pool hangs a cloud of steam, often tinted red by reflection from the waters below. [Illustration: MAMMOTH HOT SPRINGS YELLOWSTONE NATIONAL PARK] At Mammoth Hot Springs, close to Fort Yellowstone, the water bursts from the mountain-side with an enormous mineralized flow. Here lime in solution is quickly precipitated, forming basins and terraces and slopes of exquisite design, the whole adorned with intricate and fantastic fretwork of pink, brown, yellow, and white. While the deposits here are chiefly lime or travertine, those of the geysers and of the other hot springs are silica. The two kinds of deposits differ greatly. The Mammoth Hot Springs' deposits are soft and frequently change their form. The silica deposits of the geysers are hard as flint. Without this hardness, the geyser action would be impossible, as the lime and travertine formations would not withstand the explosive violence. A curious fact in this connection is that the color in and around the geysers and hot springs is in part due to the presence of algæ, a minute vegetable growth. The geyser is one of Nature's strangest freaks. These in the Yellowstone Park are the largest, most spectacular, and most artistic in the world. The geyser may be described either as a large intermittent hot-water fountain or as a small water-and-steam volcano. There are scores of these eruptive springs in the Yellowstone, and their irregularities form part of their fascination. The place and method of applying the heat, the diameter and shape of the tube, and the point of inflow and the quantity of the water are all matters affecting their activities. Apparently they, as well as the springs in general, have no underground interconnection, since the play of one geyser has no effect upon others close by.