The park region must have been for a long period the seat of volcanic action, for nearly all the rocks are cooled lavas. While the heat has disappeared from the surface, it must still be very great below, if we may judge by the quantities of hot water continually issuing from the springs. In many a subterranean cavern steam accumulates until its pressure becomes too great for the column of water occupying the channel that leads to the surface; then the water is suddenly and forcibly expelled, giving rise to a geyser eruption. When the pressure of the steam has become exhausted, the water sinks back into the earth, leaving the basin of the geyser nearly or quite empty until the steam has again collected. Each geyser has its own period of eruption and is generally very regular. One little geyser, known as the Economic, because it throws out but little water, spouts regularly about every five minutes. Other geysers are active at intervals of several hours, while some take several years to get ready for a new eruption and then spout whole rivers of boiling water. In the Upper Geyser Basin the effect is very impressive, particularly upon a cool morning. The clouds of steam and the throbbing or roaring geysers lend to the region a weird and unearthly aspect. The Yellowstone Lake is a large body of water situated almost upon the continental divide. Before the canon, or Great Falls, or even the Yellowstone River itself existed, the lake stood about one hundred and fifty feet higher than at present, and its water emptied into the Pacific Ocean instead of the Gulf of Mexico. The drainage was changed by the work of a small stream having its source in the volcanic plateau north of the lake. It deepened its channel and extended its head waters back until they tapped the lake at a point where the rim of the basin was lowest, and so drew away its waters in the opposite direction. The Yellowstone River, with its deep, wondrously colored canon and grand waterfalls, is the result of this change. [Illustration: FIG. 132.--ECONOMIC GEYSER, YELLOWSTONE PARK] To the south of Yellowstone Park, but included in one of the forest reserves, are Jackson Lake and the Teton range. The Three Tetons, one of which reaches a height of over thirteen thousand feet, were evidently noted landmarks for the hunters and trappers in the early days, for you will find them mentioned in many of the narratives of those times. The precipitous range, with its crown of jagged peaks and the beautiful lake nestling at its base, presents a picture never to be forgotten. Very different from the region which we have been studying is that embracing the Crater Lake, National Park, which is situated upon the summit of the Cascade Range in southern Oregon. Here occurred, not many thousand years ago, one of the strangest catastrophes which, so far as we know, has ever overtaken any portion of our earth.