About three miles to the southwest, farther up the Firehole River, is the Middle Geyser Basin. It is a locality covering some fifty acres, close to the river, and contains the greatest geyser in the world. The name of Hell's Half Acre was given this place in the early explorations, and still sticks. The surface is composed mainly of hot ashes, with streams of boiling water running over it. The whole basin is filled with hot springs, and surrounded by timbered hills, at the foot of which is the Prismatic Lake, its beautiful green and blue waters shading off into a deposit of bright red paint running down to the river. The great Excelsior Geyser is a fountain of enormous power but uncertain periods, which when at work throws out such immense amounts of water as to double the flow of the river. Its crater is a hundred yards wide, with water violently boiling in the centre all the time and a steady outflow. The sides of the crater are beautifully colored by the deposits, which are largely of sulphur. It is a geyser of modern origin, having developed from a hot spring within the memory of Park denizens. It throws a column over two hundred feet high, and while quiet at times for years, occasionally bursts forth, though having no fixed period. In close connection to the westward is the seething cauldron which is the immediate Hell's Half Acre, that being about its area--a beautiful but terrible lake, steam constantly rising from the surface, which boils furiously and sends copious streams over the edges. This is an uncanny spot, with treacherous footing around, and about the hottest place in the Park. THE UPPER FIREHOLE BASIN.