On approaching the source of the sound we see a large volume of steam rushing with immense velocity from an opening in the ground, while the rock around the orifice is black as jet. The guide tells us that this huge steam vent is called the Black Growler, and that it continues vigorously active summer and winter, year in and year out. Its roar can be heard four miles away. [Illustration: The Yellowstone National Park, Wyoming Beehive Geyser] The chief wonder of Lower Firehole Basin is the Great Fountain Geyser. Its formation is unique. At first sight one is led to believe that the broad circular structure which he sees is artificial. On close inspection numerous pools, moulded and nicely ornamented, are seen sunk in this stone table, while in the centre there is a large and deep pool filled with hot water, but looking like a beautiful spring. At the time of eruption this central pool of water is shot up to the height of one hundred feet or more. Near the Great Fountain Geyser is a small valley in the upper part of which is a large hot spring called the Firehole. When this spring is visited on a windless day, a light-colored flame seems to be constantly issuing from the bottom, flickering back and forth like a torch, and the visitor feels sure he is gazing at the hidden fires beneath that heat the water. It is the illusion caused by superheated steam escaping through a fissure in the rock and dividing the water. The reflection from the surface thus formed and a black background formed by the sides and bottom of the pool account for the phenomenon. Surprise Pool is found near the Great Fountain; it will make good its name should you throw into it a handful of dirt. Excelsior Geyser, not far away, is really a winter volcano, its crater being a seething caldron near the Firehole River, into which it sends six million gallons of water each day, even when not in eruption. At times it sends up a column of water, fifty feet in diameter, to the height of two hundred and fifty feet. The eruptions take place at long intervals--seven to ten years. On account of the great depth and extent of this geyser it has sometimes been denominated "Hell's Half-Acre." Following along Firehole River we pass into the Upper Basin, a section the most popular with the majority of tourists. Among the geysers in this basin we shall find Grotto, Castle, Giant, Giantess, Bee Hive, Splendid, Grand, and Old Faithful. Each of them has an interest peculiarly its own, but Old Faithful is always true to its name and is perhaps best appreciated by visitors. The opening through which Old Faithful disgorges its water is at the summit of a mound built up by its own exertions. The wrinkles on its face tell of long-continued service. Every seventy minutes this faithful worker sends up a column of water to the height of one hundred and eighty feet, and at each eruption more than one million gallons of water are thrown out.