The central spring of this group stands on the summit of a great mound built up in thin layers, by the continual but moderate overflow of the spring. The crater is twenty feet in diameter, slightly bubbling or boiling near the centre, and with a thin, elegant ring projecting a few inches over the water. Looking down into the clear water of this spring one seems to be gazing into fathomless depths, while the bright blue of the water is unequalled even by the sea. There are many such central springs, usually crowning the summits of mounds, and with projecting rims carved with an intricate delicacy truly marvellous, and adorned with colors that defy description. "The great beauty of the prismatic colors," writes Dr. Hayden, "depends much on the sunlight; about the middle of the day, when the bright rays descend nearly vertically, and a slight breeze just makes a ripple on the surface, the colors exceed comparison; when the surface is calm there is one vast chaos of colors, dancing, as it were, like the colors of a kaleidoscope. As seen through this marvellous play of colors, the decorations on the sides of the basin are lighted up with a wild, weird beauty, which wafts one at once into the land of enchantment; all the brilliant feats of fairies and genii in the Arabian Nights' Entertainments are forgotten in the actual presence of such marvellous beauty; life becomes a privilege and a blessing after one has seen and thoroughly felt its cunning skill."
Across the river, and a short distance below this group, is the largest and most imposing formation in the valley--the crater of Castle Geyser. This geyser receives its name from its resemblance to the ruins of an old castle as one enters the valley from the east. The deposited silica has crystallized in immense globular masses, like cauliflowers or spongiform corals, apparently formed about a nucleus at right angles to the centre. The entire mound is about forty feet high, and the chimney twenty feet. The lower portion rises in steps formed of thin laminæ of silica, mostly very thin, but sometimes compact, an inch or two thick. On the southeast side, where the water is thrown out continually, these steps are ornamented with the usual bead and shell work, with the large cauliflower-like masses: but the other portions are fast going to decay, and the débris are abundant. This has undoubtedly been one of the most active and powerful geysers in the basin; it still keeps up a great roaring inside, and every few moments, as observed by Dr. Hayden, it throws out a column of water to the height of ten or fifteen feet. Occasionally it seems to have more imposing eruptions, since on one occasion Lieutenant Doane saw it throw a column of water to the height of sixty feet, with the escape of heavy volumes of steam. The next year Colonel Barlow saw a similar display. According to the latter observer, the base of the crater is three hundred and twenty-five feet in circumference, and the turret one hundred and twenty-five. At the base of the turret lies a large petrified pine log, covered with a brilliant incrustation several inches thick.