"We may say, in brief, that the entire basin of the Yellowstone is volcanic. I am not prepared to pronounce it a crater, with a lake occupying the inner portion, while the mountains that surround the basin are the ruins of this great crater; but, at a period not very remote in the geological past, this whole country was a scene of wonderful volcanic activity. I regard the hot springs so abundant all over the valley as the last stages of this grand scene. Hot springs, geysers, etc., are so intimately connected with what we usually term volcanoes that their origin and action admit of the same explanation. Both undoubtedly form safety-valves or vents for the escape of the powerful forces that have been generated in the interior of the earth since the commencement of our present period; the true volcanic action has ceased, but the safety-valves are the thousands of hot springs all over this great area. I believe that the time of the greatest volcanic activity occurred during the Pliocene period--smoke, ashes, fragments of rock, and lava poured forth from thousands of orifices into the surrounding waters. Hundreds of cones were built up, fragments of which still remain; and around them were arranged by the water the dust and fragments of rock, the _ejectamenta_ of these volcanoes, in the form of the conglomerate or breccia as we find it now. These orifices may have been of every possible form--rounded or oblong, mere fissures, perhaps, extending for miles, and building up their own crater rims as the hot springs build up their rounded, conical peaks or oblong mounds at the present time." Leaving Mount Washburn, with its summit piles of basalt, and its precipitous slope scattered with agates and beautiful fragments of sardonyx, chalcedony, and malachite, let us descend to the valley. The trail pursues a tortuous way to avoid the fallen timber and the dense groves of pine, descending the almost vertical inner sides of the rim of the Yellowstone Basin, to the valley of a small creek. Two or three miles down this stream is a hideous glen, filled with sulphurous vapor emitted from six or eight boiling springs of great size and activity. Mr. Langford says of this unsavory place: