The extreme north of the Lower Basin is bounded by the East Fork of Madison River, along whose valley, within the basin and above it, are numerous groups of interesting springs, though not materially different from those already described. Near the head of the stream sulphur springs are abundant, with here and there extensive deposits of sulphur. Steam-vents are frequent, their orifices lined with sulphur, and the surrounding crust filled with crystals of a vivid yellow. The channels of the streams are lined with a delicate veil of creamy sulphur. In some of the springs, lower down the stream, iron predominates. Within the basin on the south side of the East Fork are a hundred springs or more, any one of which, if alone, would be worthy of elaborate description. In some the fallen leaves of trees are frosted with silica as white as snow, and the inner surface of the basins are covered with a delicate bead-like embroidery of marvellous beauty. The most beautiful of the group is a Prismatic Spring, like those described in the Upper Basin. "Nothing ever conceived by human art," says Dr. Hayden, "could equal the peculiar vividness and delicacy of coloring of these remarkable prismatic springs." About a mile south of the East Fork, at the head of a little stream that flows into the Firehole, is another of these brilliant springs. A thin, delicately ornamental rim of silica surrounds a basin six feet in diameter, filled to the brim with water of marvellous transparency. When its surface is rippled by a passing breeze, the reflected sunlight is broken as by a million prisms. It is called the Rainbow Spring. CHAPTER XIV. NATURAL HISTORY OF GEYSERS AND OTHER THERMAL SPRINGS. In Icelandic speech the word _geyser_ means simply _rager_, and is applied indiscriminately to all turbulent fountains of water or mud. The most violent and noisy "rager" on the island being the great intermittent spouting spring near Haukadal, it naturally gained for itself the title, The Geyser; and being the earliest known and most remarkable fountain of the kind, its native common name was adopted in other languages as the generic name for all springs of its class.