HOT SPRINGS AND GEYSERS. 



719 



or quartz, or siliceous earth, from the silica set free. Carbonic acid 

 is sometimes given out in such places, when there is limestone below 

 to be decomposed, — some acid (either sulphuric acid or silica in 

 solution) setting free the carbonic acid, by combining with the lime. 



2. Hot springs. — Hot springs are common in volcanic regions. 

 The waters may be either essentially pure, or strong mineral solutions. 



In many cases, the hot waters hold silica in solution, whose depo- 

 sition, over the region around, makes irregular accumulations of a 

 coarse opal, or rarely of quartz, often in the form of low cones or 

 rims about basins, and sometimes in irregular massive inclosures. 

 Occasionally, the waters are calcareous, instead of siliceous, and make 

 calcareous basins or cones. The waters get their silica from the 

 rock adjoining, and mostly from its feldspar, this mineral containing, 

 besides silica, the alkalies that are needed to aid in dissolving it. 

 The lime comes from limestones, as already explained. 



Iceland has long been noted for its geysers ; but it is far out- 

 stripped by the region of the Yellowstone Park, explored and mapped 

 by the expeditions under the charge of Dr. F. V. Hayden. This 

 locality is situated about the head-waters of the Yellowstone and 

 Madison, two tributaries of the Missouri, and of the Snake River, a 

 tributary of the Columbia, at heights of 6,500 to 8,000 feet above the 

 sea-level. The geysers, which are mostly about the Fire-Hole Fork 

 of the Madison, and near Shoshone Lake at the head of Lake Fork of 

 the Snake, are exceedingly numerous, and play at all heights, up to 200 

 feet, or more ; and, besides, there are multitudes of hot springs of 

 various temperatures, the most of them between 160° and 200° F., the 

 boiling-point of the region being 198° to 199° F. All together, the 

 number of hot vents in this region cannot be less than 10,000. But 

 the region is far from fully explored ; and the geyser-areas east and 

 southeast of Yellowstone Lake, recently reported, may double this 

 number. 



Figs. 1118-1120. 



Geyser-Cones. — Fig. 1118, Giant Geyser; 1119, Liberty Cap; 1120, Beehive Geyser. 



The hot waters of the Fire-hole Fork of the Madison and of the Shoshone Lake region 

 are siliceous, while those of Gardiner's River, a tributary of the Yellowstone, are 

 calcareous. Some of the forms of the geyser-cones are shown in the accompanying 

 figures. Fig. 1118 represents the cone of the "Giant" Geyser, in the Upper Geyser 

 Basin of the Fire-hole; it is about ten feet high and twenty -four feet in diameter at 



