Two hundred yards below the Fan are two lively geysers called the Sentinels. The one on the right bank of the stream is in constant agitation, its waters revolving horizontally with great violence, occasionally spouting upward to the height of twenty feet, with a lateral projection of fifty feet. Much steam is thrown off at each eruption. The crater of this geyser is three feet by ten. The companion Sentinel on the other side of the stream is smaller and less active. At this point the river-valley is narrow and the stream rapid, with a considerable fall. Forty or fifty comparatively unimportant geysers and boiling springs are scattered along the narrow valley to the junction of Iron Spring Creek, the lower limit of the Upper Geyser Basin.
Iron Spring Creek, a stream about half the size of Firehole River, takes its name from a group of springs on its banks, about a mile south of the Giant. Among the most noticeable of these is a group of eight beautiful springs enclosed in a single rim, one hundred and forty feet in length. The interior of the basin is lined with a rose-colored deposit. These springs are situated on the crest of an eminence incrusted with rocky deposits which encroach on the adjacent forest, whose dead and whitened trunks bear evidence of the deadly effect of the hot water flowing among them. On a considerable mound, at the junction of Iron Creek with the main stream, is a group of geysers that do not differ materially from those already described. The central member of the group is known as Soda Geyser.
THE BEE-HIVE.
CHAPTER XIII.
LOWER GEYSER BASIN.—FIREHOLE RIVER.
Between the Upper and Lower Geyser Basin is a space of two or three miles entirely free from hot springs; yet the abundance of spring deposit over all the valley shows that the region was once the scene of great thermal activity; the bottom over which the river flows is paved with silica. Vegetation grows remarkably rank along the stream, and in the valley where the crust of silica does not prevent it, the perpetual warmth caused by the proximity to the springs being very favorable to the growth of plants. The forest grows close down to the margin of the river, making travel very difficult, and in one place the hills of trachyte almost close in the valley.