Although none of these French volcanoes have been in activity within the period
of history or tradition, their forms are often very perfect. Some, however, have
been compared to the mere skeletons of volcanoes, the rains and torrents having
washed their sides, and removed all 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 perpendicular walls, or DIKES, as they are
called, of volcanic rock, which have burst through the other materials. Such
dikes are also observed in the structure of Vesuvius, Etna, and other active
volcanoes. 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 showering down from the air, or
incumbent waters, of sand and cinders, first shot up from the interior of the
earth by the explosions of volcanic gases.

Besides the parts of France above alluded to, there are other countries, as the
north of Spain, the south of Sicily, the Tuscan territory of Italy, the lower
Rhenish provinces, and Hungary, where spent volcanoes may be seen, still
preserving in many cases a conical form, and having craters and often lava-
streams connected with them.

There are also other rocks in England, Scotland, Ireland, and almost every
country in Europe, which we infer to be of igneous origin, although they do not
form hills with cones and craters. Thus, for example, we feel assured that the
rock of Staffa, and that of the Giant's Causeway, called basalt, is volcanic,
because it agrees in its columnar structure and mineral composition with streams
of lava which we know to have flowed from the craters of volcanoes. We find also
similar basaltic and other igneous rocks associated with beds of TUFF in various
parts of the British Isles, and forming DIKES, such as have been spoken of; and