In case of natural geysers a considerable quantity of heat escapes between eruptions in steam which issues quietly from the bowl of the geyser. If this heat be retained by plugging the mouth of the tube with a barrowful of turf, as is sometimes done with the geyser _Strokr_ in Iceland, eruption is promoted and so takes place earlier. Another method of securing the same result is to increase the viscosity of the water through the addition of soap, as was accidentally discovered by a Chinaman who was utilizing the geyser water in the Yellowstone Park for laundry operations. After this discovery it became a common custom to “soap” the Yellowstone geysers in order to make them play; but this method was prohibited under heavy penalty after the disastrous eruption of the Excelsior Geyser. =The deposition of siliceous sinter by plant growth.=—Geysers are known only from areas of siliceous volcanic lava, and this may perhaps have its cause in the easier solution of the geyser tube from such materials. The silica dissolved in the heated waters is _again_ deposited at the surface to form _siliceous sinter_ or _geyserite_. This material forms terraces surrounding the geysers or is built up into mounds which are often quite symmetrical, such as those of the Bee Hive and Lone Star geysers of the Yellowstone Park (Fig. 204). [Illustration: FIG. 204.—Cone of siliceous sinter built up about the mouth of the Lone Star Geyser in the Yellowstone National Park.] The greater part of this separation of silica from the heated geyser waters is due to the action of plants or algæ that are able to grow in the boiling waters and which produce the beautiful colors in the linings to the hot springs. The wonderful variety of the tints displayed is accounted for by the fact that the algæ take on different colors at different temperatures. The silica is deposited from the water in the gelatinous hydrated form, which, however, dries in the sun to a white sand. The growth within the pools goes on in a manner similar to that of a coral reef, the algæ dying below and there becoming encased in the rock lining while still continuing to grow upon the surface. Whereas sinter of this nature, when deposited by evaporation alone, can produce a maximum thickness of layer of a twentieth of an inch each year, the growth from alga deposition within limited areas may be as much as eight inches during the same period. READING REFERENCES FOR CHAPTER XIV General:— F. H. KING. Principles and Conditions of the Movements of Ground Water, 19th Ann. Rept. U. S. Geol. Surv., 1899, Pt. ii, pp. 59-294, pls. 6-16. C. S. SLICHTER. The Motions of the Underground Waters, Water Supply Paper No. 67, U. S. Geol. Surv., 1902, pp. 1-106, pls. 1-8; Field Measurements of the Rate of Movement of Underground Waters, _ibid._, No. 140, 1905, pp. 1-122, pls. 1-15.