Three notable features of similarity in these geyser regions are the presence of volcanic rocks of remote or recent origin; proximity to the earth's surface of active sources of subterranean heat; and the presence of a great number of lakes. In all three cases, lava, heat and water are the characteristic geologic and physical accompaniments of those particular phenomena which will now be described more in detail. CHAPTER III. GEYSERS. The hot springs of the Yellowstone National Park may be roughly divided into two classes, eruptive and non-eruptive. To the first the term _geyser_ is applied, while the term _hot springs_ is restricted to the second. These two classes pass into each other by insensible gradations and the line of demarcation it is not possible to draw. The following description will pertain only to those examples about which there is no doubt, and which may be taken as types of their class. A geyser may be defined as a periodically eruptive hot spring. The name, as might be expected, is of Icelandic origin, and comes from the verb _geysa_, _to gush_. The general characteristics of a true geyser, as illustrated by the most perfect example known, Old Faithful in the Yellowstone Park, are the following: (1.) There is an irregular tube descending from the earth's surface to some interior source of heat. (2.) The mouth of this tube may be either a self-built mound or cone (as in the example), or simply an open pool. (3.) Into this tube meteoric water finds its way and is subjected to the action of heat. (4.) The result is an eruption and expulsion of the water from the tube with more or less violence. (5.) The eruption is generally preceded by slight preliminary upheavals leading gradually to the final outburst. (6.) After cessation of the eruption there is usually a considerable escape of steam. (7.) A quiescent period, generally of indeterminate duration, follows during which the conditions necessary for an eruption are reproduced. Geyser phenomena have attracted a great deal of scientific attention, and many theories have been advanced to explain them. Passing over for the present the various less important views, attention will first be given to Bunsen's theory, because it is, upon the whole, the most satisfactory explanation yet advanced. This theory was a direct deduction from observations upon the Great Geyser of Iceland, and has been experimentally illustrated by artificial examples.