Water may descend to depths from which it can never be brought back by hydrostatic pressure.  It is absorbed by highly heated rocks deep below the surface.  From time to time some of this deep-seated water may be returned to open air in the steam of volcanic eruptions.

Surface deposits of springs.  Where subterranean water returns to the surface highly charged with minerals in solution, on exposure to the air it is commonly compelled to lay down much of its invisible load in chemical deposits about the spring.  These are thrown down from solution either because of cooling, evaporation, the loss of carbon dioxide, or the work of algae.

Many springs have been charged under pressure with carbon dioxide from subterranean sources and are able therefore to take up large quantities of lime carbonate from the limestone rocks through which they pass.  On reaching the surface the pressure is relieved, the gas escapes, and the lime carbonate is thrown down in deposits called travertine.  The gas is sometimes withdrawn and the deposit produced in large part by the action of algae and other humble forms of plant life.

At the Mammoth Hot Springs in the valley of the Gardiner River, Yellowstone National Park, beautiful terraces and basins of travertine 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.

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.