fumarole clay, the material of the mud-pools. After the sulphurous acid comes sulphuretted hydrogen, produced by the action of steam on sulphids; and by the mutual decomposition of sulphuretted hydrogen and sulphurous acid sulphur is formed, the characteristic precipitate in all solfataras, while the deposition of silica is either entirely wanting or quite inconsiderable, and the smell of sulphuretted hydrogen is but rarely noticed. These acid springs have no periodical outbursts of water. In course of time the source of sulphurous acid becomes exhausted, and sulphuretted hydrogen alone remains active. The acid reaction of the soil disappears, yielding to an alkaline reaction by the formation of sulphids. At the same time carbonic acid begins to act upon the rocks, and the alkaline bi-carbonates thus produced dissolve the silica, which on the evaporation of the water is deposited in the form of opal or quartz or silicious earth, and thus the shell of the springs is formed, on the structure of which the periodicity of the outburst depends.... The deposition of silica in quantities sufficient for the formation of this spring-apparatus in the course of years, takes place only in the alkaline springs. Their water is either neutral or has a slightly alkaline reaction. Silica, common salt, carbonates and sulphates are the chief ingredients dissolved in it. In the place of sulphurous acid the odor of sulphuretted hydrogen is sometimes observed in these springs.... By the gradual cooling of the volcanic rocks under the surface of the earth the hot springs themselves gradually die out, for they too are but a transient phenomenon in the eternal change of created things." CHAPTER XV. MR. EVERTS'S THIRTY-SEVEN DAYS OF PERIL.