The antiquity of these springs is a question of great interest, yet difficult of solution. When were these immense deposits begun? On the margin of the mountain, high above the present position of the hot springs, is a bed of white, or yellowish white limestone, from fifty to a hundred and fifty feet thick. It is regularly stratified and the jointing is complete. There is a belt a mile long and one fourth of a mile wide, covered with cubical masses of this rock that have fallen down the slope of the mountain. These immense blocks, fifty to one hundred feet in each dimension, appear as if the mass had slowly fallen down as the underlying rocks were worn away. So thickly is this belt covered with these huge masses that it is with the greatest difficulty one can walk across it. It would seem that this bed must at one time have extended over a portion or all of the valley of Gardiner's River. Much of the rock is very compact, and would make beautiful building-stone, on account of its close texture and color, and it could be converted into the whitest of lime. If the rocks are examined, however, over a considerable area, they are found to possess all the varieties of structure of a hot-spring deposit. Some portions are quite spongy, and decompose readily; others are made up of very thin laminæ, regular or wavy; enough to show the origin of the deposit without a doubt. But in what manner was it formed? Dr. Hayden believes that the limestone was precipitated in the bottom of a lake, which was filled with hot-springs, much as the calcareous matter is laid down in the bottom of the ocean at the present time. Indeed, portions of the rock do not differ materially from the recent limestones now forming in the vicinity of the West India Islands. The deposit was evidently laid down on a nearly level surface, with a moderately uniform thickness, and the strata are horizontal. Since this group of strata was formed, the country has been elevated, and the valley of Gardiner's River has been carved out, so that the commencement of the period of activity of these springs must date back to a period merging on, but just prior to, the present geological period--probably at the time of the greatest action of the volcanic forces in this region. Classed with reference to their chemical constituents, the springs here and elsewhere in the Yellowstone Valley are of two kinds: those in which lime predominates, and those in which silica is most abundant. The springs of Gardiner's River are mainly the former. Where does the lime come from? The geology of the country surrounding the springs shows already that there is underneath the spring deposits, at least a thickness of 1,500 feet, of carboniferous limestone; and if the origin of the heat which so elevates the temperature of the waters of these springs is as deep seated as is generally supposed, the heated waters have ample time and space for dissolving the calcareous rocks through which they flow. CHAPTER VI.