the walls of which are formed of thick stone, free from penetrating clefts. Water-ways entering it from above and from the sides bring in cool waters; while other streams bring up from below water heated to above the boiling-point, or steam from the volcanic foci. The inferior conducting power of the rock-masses, inclosing the chamber
Fig. 2.—Mackenzie's Theory of Eruption.
and the stream-channels, prevents any material diminution of the temperature of the steam and the water, so that the chamber, b, becomes filled with hot water and steam. By virtue of its levity, the latter collects in the upper part of the chamber, while the water covers the bottom, and at the same time, as it is added to by the constant inflow from the canals which enter the chamber, b, rises and shuts up the tube at a. The steam is thus deprived of an outlet, and, since it is continually compressed into a smaller space by the constantly increasing mass of water, is added to by the entrance of new steam, and is heated to a higher temperature by the heat brought in with the hot water that keeps flowing in; it on its side exercises upon the surface of the water in the chamber a pressure that, increasing every moment, gradually raises the water in the tube, c, and causes an overflow over the rim of the basin, d. Finally, the pressure of the inclosed steam becomes so powerful as to overcome the absolute weight of the mass of water in the tube, c, and throw it up strongly and suddenly in fountain-like spouts. After the steam in b has relieved itself, and the pressure on the water has thereby been diminished, a becomes again closed up by the rushing back of the water from the tube and the flow of water from the chamber, and the conditions requisite to another eruption are produced. The temperature of the water in the chamber and in the tube rises continually between one eruption and another, and reaches in many places the degree at which steam is formed. This is the case in the lower part of the tube at a, where, the water nearer the chamber having a higher temperature than in other parts of c, the steam rises in c and causes the bubblings which appear in the basin at intervals of from one and a half to two hours. This theory, which supposes a subterranean cavity acting as a kind
