losing its acid on exposure to the air, leaves a deposit known as "travertine," sometimes in great masses. In the Auvergne, the travertine exists in large rocks which take the form of natural aqueducts and bridges; in Carlsbad it has filled the whole bottom of the valley, and lies under the foundations of the town; and in Rome it has furnished the stone for St. Peter's and all the principal buildings. When springs charged with silica or carbonate of lime appear upon the slope of a hill composed of loose volcanic materials, they give rise to the remarkable structures known as sinter and travertine-terraces. The water flowing downward from the vent forms a hard deposit upon the lower slope of the hill, while the continual deposition of solid materials within the vent tends to choke it up. As a new vent can not be forced by the waters through the hard rock formed below, it is opened a little higher up (Fig. 10). Thus the site of the spring is gradually
Fig. 10.—Diagram illustrating the Mode of Formation of Travertine and Sinter-Terraces on the Sides of a Hill of Tuff.
shifted farther and farther back into the hills. As deposition takes place along the surfaces over which this water flows, terraces are built up inclosing basins. Of structures of this kind we have remarkable examples in the sinter-terraces of Rotomahana in New Zealand, and the travertine-terraces of Gardiner's River in the Yellowstone Park.
We sometimes find examples of volcanoes which by the action of denuding forces have had their very foundations exposed to view. Such examples occur in the Western Islands of Scotland, where we are able to trace the ground-plan of the volcanic pile, and study the materials which have consolidated deep beneath the surface in the very heart of the mountain. An admirable specimen of them is given by the "dissected volcano" forming the Island of Mull, which was probably originally nearly thirty miles in diameter at its base, and ten or twelve thousand feet high, but is now reduced to a group of hills few of which are more than three thousand feet high. Here, as shown in the figure representing the ground-plan (Figs. 11 and 12), great masses of granite, syenite, and diorite—the crystalline representa-
