Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 2.djvu/22

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Our section represents three cases of Volcanic craters; the most simple (i. 5.) rising through Granite, or stratified rocks, at the bottom of the sea, and accumulating craters, which, like those of Lipari and Stromboli, Sabrina, and Graham Islands, are occasionally formed in various parts of the ocean. The second case is that of volcanoes, which, like Etna and Vesuvius, are still in action on the dry land, (i. 1. to i. 4.) The third is that of extinct volcanoes, like those in Auvergne, (h1. h2.) which, although there exist no historical records as to the period of their last eruptions, show by the perfect condition of their craters, that they have been formed since the latest of those aqueous inundations, that have affected the Basalts and Tertiary strata, through which they have burst forth.

One great difference between the more ancient Basaltic eruptions and those of the Lava and Trachyte of existing volcanoes, is that the emission of the former, probably taking place under the pressure of deep water, was not accompanied by the formation of any permanent craters.

In both cases, the fissures through some of which these Eruptions may have issued, are abundantly apparent under

the upward passage of the Lava through fractures in the sordid Granite.

At Graveneire, near Clermont, a stream of Lava still retains the exact form, in which it issued through a fissure in the side of a mountain of Granite, and overflowed the subjacent valley. Most accurate representations of this, and many similar productions of Volcanic Eruptions from the Granite of this District may be seen in Mr. Poulett Scrope's inimitable Panoramic Views of the Volcanic formations of Central France.