A half mile south of these springs we found an alum spring yielding but little water and surrounded with beautiful alum crystals. From its border we obtained a great many curiously shaped deposits of alum slightly impregnated with iron. The border of this spring below the surface had been undermined in many places by the violent boiling of the water, to the distance of several feet from the margin, so that it was unsafe to stand near the edge of the spring. This, however, I did not at first perceive; and, as I was unconcernedly passing by the spring, my weight made the border suddenly slough off beneath my feet. General Washburn noticed the sudden cracking of the incrustation before I did, and I was aroused to a sense of my peril by his shout of alarm, and had sufficient presence of mind to fall suddenly backwards at full length upon the sound crust, whence, with my feet and legs extended over the spring, I rolled to a place of safety. But for General Washburn's shout of alarm, in another instant I would have been precipitated into this boiling pool of alum. We endeavored to sound the depth of this spring with a pole twenty-five feet long, but we found no bottom. Everything around us--air, earth, water--is impregnated with sulphur. We feel it in every drop of water we drink, and in every breath of air we inhale. Our silver watches have turned to the color of poor brass, tarnished. General Washburn and I again visited the mud vulcano to-day. I especially desired to see it again for the one especial purpose, among others of a general nature, of assuring myself that the notes made in my diary a few days ago are not exaggerated. No! they are not! The sensations inspired in me to-day, on again witnessing its convulsions, and the dense clouds of vapor expelled in rapid succession from its crater, amid the jarring of the earth, and the ominous intonations from beneath, were those of mingled dread and wonder. At war with all former experience it was so novel, so unnaturally natural, that I feel while now writing and thinking of it, as if my own senses might have deceived me with a mere figment of the imagination. But it is not so. The wonder, than which this continent, teeming with nature's grandest exhibitions, contains nothing more marvelous, still stands amid the solitary fastnesses of the Yellowstone, to excite the astonishment of the thousands who in coming years shall visit that remarkable locality.[J]