. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 594 VOLCANIC KOCKS. [Ch. XXVIII. constitute the most striking peculiarity of this class of formations. Many hundreds of these cones are seen in central France, in the an- cient provinces of Auvergne, Yelay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 594 VOLCANIC KOCKS. [Ch. XXVIII. constitute the most striking peculiarity of this class of formations. Many hundreds of these cones are seen in central France, in the an- cient provinces of Auvergne, Yelay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and following the lowest levels of the existing valleys. The origin of the cone and crater-shaped hill is well under- Eig. Part of the chain of extinct volcanoes called the Monts Dome, Auvergne. (Scrope.) stood, the growth of many having been watched during volcanic erup- tions. A chasm or fissure first opens in the earth, from which great volumes of steam and other gases are evolved. The explosions are so violent as to hurl up into the air fragments of broken stone, parts of which are shivered into minute atoms. At the same time melted stone or lava usually ascends through the chimney or vent by which the gases make their escape. Although extremely heavy, this lava is forced up by the expansive power of entangled gaseous fluids, chiefly steam or aqueous vapor, exactly in the same manner as water is made to boil over the edge of a vessel when steam has been generated at the bottom by heat. Large quantities of the lava are also shot up into the air, where it separates into fragments, and acquires a spongy texture by the sudden enlargement of the included gases, and thus forms scoria, other portions being reduced to an impalpable powder or dust. The showering down of the various ejected materials round the orifice of eruption gives rise to a conical mound, in which the successive envelopes of sand and scoriae form layers, dipping on all sides from a central axis. In the mean t
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
