Elements of geology / by Charles Lyell.

  • Charles Lyell
Date:
1838
Catalogue details

Licence: Public Domain Mark

Credit: Elements of geology / by Charles Lyell. Source: Wellcome Collection.

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    more or less perfect on many of their summits. These cones are composed moreover of lava, sand, and ashes, similar to those of active volcanos. Streams of lava may sometimes be traced proceed- ing from the cones into the adjoining valleys, where they choke up the ancient channels of rivers with solid rock, in the same manner as some modern flows of lava in Iceland have been known to do, the rivers either flowing beneath or cutting out a narrow passage on one side of the lava. Although none of these French volcanos have been in activity within the period of history or tradition, their forms are often very perfect. Some how- ever have been compared to the mere skeletons of volcanos, the rains and torrents having washed their sides, and removed ail the loose sand and scoriae, leaving only the harder and more solid materials. By this erosion, and by earthquakes, their internal structure has occasionally been laid open to view, in fissures and ravines; and we then behold not only many successive beds and masses of porous lava, sand, and scoriae, but also per- pendicular walls, or dikes, as they are called, of volcanic rock, cutting through the other materials. Such dikes are also observed in the structure of Vesuvius, Etna, and other active volcanos. They have been formed by the pouring of melted matter, whether from above or below, into open fissures, and they commonly traverse deposits of volcanic tuff, a substance produced by the shower-
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