At the Mammoth Hot Springs in the valley of the Gardiner River, Yellowstone National Park, beautiful terraces and basins of travertine (Fig. 35) are now building, chiefly by means of algae which cover the bottoms, rims, and sides of the basins and deposit lime carbonate upon them in successive sheets. The rock, snow-white where dry, is coated with red and orange gelatinous mats where the algae thrive in the over-flowing waters. Similar terraces of travertine are found to a height of fourteen hundred feet up the valley side. We may infer that the springs which formed these ancient deposits discharged near what was then the bottom of the valley, and that as the valley has been deepened by the river the ground water of the region has found lower and lower points of issue. In many parts of the country calcareous springs occur which coat with lime carbonate mosses, twigs, and other objects over which their waters flow. Such are popularly known as petrifying springs, although they merely incrust the objects and do not convert them into stone. Silica is soluble in alkaline waters, especially when these are hot. Hot springs rising through alkaline siliceous rocks, such as lavas, often deposit silica in a white spongy formation known as _siliceous sinter_, both by evaporation and by the action of algae which secrete silica from the waters. It is in this way that the cones and mounds of the geysers in the Yellowstone National Park and in Iceland have been formed (Fig. 234). Where water oozes from the earth one may sometimes see a rusty deposit on the ground, and perhaps an iridescent scum upon the water. The scum is often mistaken for oil, but at a touch it cracks and breaks, as oil would not do. It is a film of hydrated iron oxide, or _limonite_, and the spring is an iron, or chalybeate, spring. Compounds of iron have been taken into solution by ground water from soil and rocks, and are now changed to the insoluble oxide on exposure to the oxygen of the air. In wet ground iron compounds leached by ground water from the soil often collect in reddish deposits a few feet below the surface, where their downward progress is arrested by some impervious clay. At the bottom of bogs and shallow lakes iron ores sometimes accumulate to a depth of several feet. Decaying organic matter plays a large part in these changes. In its presence the insoluble iron oxides which give color to most red and yellow rocks are decomposed, leaving the rocks of a gray or bluish color, and the soluble iron compounds which result are readily leached out,--effects seen where red or yellow clays have been bleached about some decaying tree root. The iron thus dissolved is laid down as limonite when oxidized, as about a chalybeate spring; but out of contact with the air and in the presence of carbon dioxide supplied by decaying vegetation, as in a peat bog, it may be deposited as iron carbonate, or _siderite_.