The heating of geyser and hot-spring water must cool the lava or other source of heat below. As this takes place, the time between eruptions becomes longer and longer. In the course of time, therefore, the geyser must cease to be eruptive, and when this change is brought about the geyser becomes a hot spring. Within historic times several geysers have ceased to erupt and new ones have been developed. In the Yellowstone Park, where there are said to be something like 3000 vents of all sorts, hot springs which are not eruptive greatly outnumber the geysers. From many of the vents but little steam issues, and from some, little else. [Illustration: +Fig.+ 218.—Hot springs deposits. Terraces of Mammoth Hot Springs, Yellowstone Park.] A few geysers have somewhat definite periods of eruption. Of such “Old Faithful” is the type; but even this geyser, which formerly erupted at regular intervals of about an hour, is losing the reputation on which its name is based. Not only is its period of eruption lengthening, but it is becoming irregular, and the irregularity appears to be increasing. In the short time during which this geyser has been under observation its period has changed from a regular one of sixty minutes, or a little less, to an irregular one of seventy to ninety minutes. In the case of some geysers years elapse between eruptions, and in some the date of the last eruption is so distant that it is uncertain whether the vent should be looked upon as a geyser or merely a hot spring. In the Yellowstone Park[120] the geysers are mainly in the bottoms of valleys (Fig. 219), but the deposits characteristic of geysers are found in not a few places well above the present bottoms. These deposits record the fact that in earlier times the geysers were at higher levels than now. It is probable that they have been, at all stages in their history, near the bottoms of the valleys, and that, as the valleys have been deepened the ground-water has found lower and lower points of issue. In this respect the geysers have probably had the same history as other springs. [Illustration: +Fig.+ 219.—Hot springs and geysers. Norris Geyser basin, Yellowstone Park.] Unless new intrusions of lava occur, or unless heat is otherwise renewed at the proper points, it is probable that all existing geysers will become extinct within a time which is, geologically, short. New geyser regions may, however, develop as old ones disappear. =Artesian wells.=—Originally the terms _artesian_ wells and _flowing_ wells were synonymous; but at the present time any notably deep well is called _artesian_, especially if it descends to considerable depths below the mantle rock. The artesian well which does not flow, does not differ from common wells in principle; but being deeper, the water which it affords is often more thoroughly filtered and frequently more highly mineralized than that of other wells. The flowing well is really a gushing spring, the opening of which was made by man.