Glacial action and the common agents of denudation have given the Park country its present general aspect. These later modifications have indeed been extensive, and the great variety of form now seen in the valleys, cañons and hills is the result of their combined action. The Yellowstone Cañon is a marked example of erosion on a large scale. A direct result of its formation was the partial draining of Yellowstone Lake, which had previously existed at a much higher level than now, and spread over the entire area of the present Hayden Valley. Since the cessation of the basaltic lava flows there seem to have been no further lava outpourings in this region. The old volcanoes have been long extinct and their craters have been modified almost beyond recognition. But evidences of the power which once worked beneath them are still abundant, although no longer on so imposing a scale. It is the hot springs and geysers still in existence which partly render this region so widely celebrated. That this thermal action originates mainly in the same source of energy which once poured out the vast fields of lava, there is no reason to doubt. Many plausible explanations are advanced to account for the existence of subterranean heat, but whatever may be its real origin it is doubtless the same for both classes of phenomena. The action which is now observable has continued in an ever-decreasing degree since the close of the lava period. Over vast tracts of the Park plateau, the rocks are entirely decomposed to unknown depths by the ascending superheated vapors. Some idea of the extent of this action may be obtained at the Grand Cañon, which has cut its way a thousand feet downward into the decomposed volcanic rock without yet reaching its bottom. The infinite variety of chemic products resulting from this decomposition has given the Cañon its wonderful coloration. The same condition largely prevails over the Park plateau. Where now are dense forests and no superficial evidence of unusual conditions, there will frequently be found, by digging beneath the surface, the familiar proof that thermal activity once prevailed there. In constructing the tourist route from the Upper Geyser Basin to the Yellowstone Lake, where for nearly the whole distance there is a complete absence of hot springs, the evidences of former volcanic activity were found to be abundant.