The earliest volcanic eruptions occurred in the later Eocene epoch and were continued into the succeeding Miocene stage. These consisted of rocks moderately rich in silica, and are grouped under the heads of propylite and andesite. To these succeeded during the Pliocene epoch still more highly silicated rocks of trachytic type, consisting of sanidine and oligoclase trachytes. Then came eruptions of rhyolite during the later Pliocene and Pleistocene epochs; and lastly, after a period of cessation, during which the rocks just described were greatly eroded, came the great eruptions of basaltic lava, deluging the plains, winding round the cones or plateaux of the older lavas, descending into the river valleys and flooding the lake beds, issuing forth from both vents and fissures, and continuing intermittently down almost into the present day--certainly into the period of man's appearance on the scene. Thus the volcanic history of Western America corresponds remarkably to that of the European regions with which we have previously dealt, both as regards the succession of the various lavas and the epochs of their eruption. (_i._) _The Yellowstone Park._--The geysers and hot springs of the Yellowstone Park, like those in Iceland and New Zealand, are special manifestations of volcanic action, generally in its secondary or moribund stage. The geysers of the Yellowstone occur on a grand scale; the eruptions are frequent, and the water is projected into the air to a height of over 200 feet. Most of these are intermittent, like the remarkable one known as Old Faithful, the Castle Geyser, and the Giantess Geyser described by Dr. Hayden, which ejects the water to a height of 250 feet. The geyser-waters hold large quantities of silica and sulphur in solution, owing to their high temperature under great pressure, and these minerals are precipitated upon the cooling of the waters in the air, and form circular basins, often gorgeously tinted with red and yellow colours.[11] [1] J. W. Powell, _Exploration of the Cañons of the Colorado_, pp. 114, 196. Major Powell describes a fault or fissure through which floods of lava have been forced up from beneath and have been poured over the surface. Many cinder-cones are planted along the line of this fissure. [2] Capt. C. E. Dutton. _Sixth Ann. Rep. U.S. Geol. Survey_, 1884-85. [3] Dutton, _loc. cit._, chap. iv. p. 165. [4] _Amer. Jour. Science_, vol. 3., ser. (1871). A beautiful map of this mountain is given in the _Fifth Annual Report, U.S. Geol. Survey_, 1883-84. Plate 44. [5] Daubeny, _loc. cit._, p. 474. [6] Gilbert, _Monograph U.S. Geol. Survey_, vol. i. (1890). [7] Powell, _Exploration of the Colorado River_, p. 177, etc. (1875). Hayden, _Rep. U.S. Geol. Survey of the Colorado, etc._ (1871-80). [8] Richthofen, _Natural System of Volcanic Rocks_, Mem. California Acad. Sciences, vol. i. (1868). [9] Geikie, _Geological Sketches at Home and Abroad_, p. 271 (1882).