30 
ELEMENTS OF GEOLOGY. 
lava, sand, and ashes, similar to those of active volcanoes. 
Streams of lava may sometimes be traced from the cones 
into the adjoining valleys, where they have choked np the 
ancient channels of rivers with solid rock, in the same man¬ 
ner 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 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 san^ and scoriae, leav¬ 
ing only the harder and more solid materials. By this ero¬ 
sion, and by earthquakes, their internal structure has occa¬ 
sionally 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 pro¬ 
duced 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 pre¬ 
serving 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 Stafia, and that of the Giant’s Causeway, called ba¬ 
salt, 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 some of the strata 
through which these dikes cut are occasionally altered at 
the point of contact, as if they had been exposed to the in¬ 
tense heat of melted matter. 
