VOLCANIC PRODUCTS. 



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losing its acid on exposure to the air, leaves a deposit known as " trav- 

 ertine," sometimes in great masses. In (he 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 flow- 

 ing 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-Ter- 

 races 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- 



