Hot-spring deposits and algae [Illustration: MOUND OF SINTER at Castle Geyser, Upper Geyser Basin. Lower part of mound has well-defined layers probably deposited by normal hot springs. The upper, irregular part resulted from the vigorous eruptions characteristic of geysers and marks a change in the local hot-spring activity. (Fig. 47)] Nearly all geysers and many hot springs build mounds or terraces of mineral deposits; some are so unusual in form that descriptive names have been given to them, such as Castle Geyser (fig. 47). These deposits are generally made up of many very thin layers of rock. Each layer represents a crust or film of rock-forming mineral which was originally dissolved in hot water as it flowed through the underground rocks, and which was then precipitated as the water spread out over the surrounding ground surface. [Illustration: TERRACES OF TRAVERTINE at Opal Springs, Mammoth Hot Springs area. (Fig. 48)] [Illustration: Closeup view shows layered and porous nature of the travertine.] In all major thermal areas of the Park, with the exception of Mammoth Hot Springs, most of the material being deposited is _sinter_ (the kind found around geysers is popularly called _geyserite_). Its chief constituent is silica (the same as in quartz and in ordinary window glass). At Mammoth, the deposit is _travertine_ (fig. 48), which consists almost entirely of calcium carbonate. The material deposited at any given place commonly reflects the predominant kind of rock through which the hot water has passed during its underground travels. At Mammoth Hot Springs the water passes through thick beds of limestone (which is calcium carbonate), but in other areas the main rock type through which the water percolates is rhyolite, a rock rich in silica. Through centuries of intense activity, layers of sinter have built up on the floors of the geyser basins (fig. 44); these deposits are generally less than 10 feet thick. In one drill hole at Mammoth, deposits of travertine extend to a depth of 250 feet. Dead trees and other kinds of vegetation whose life processes have been choked off by the heat, water, and precipitated minerals of hot-spring activity are a common sight in many places (fig. 51).