6 VOLCANIC ROCKS. [Ch. 1 



of active volcanoes. Streams of lava may sometimes be tiaced from tlie 

 cones into the adjoining valleys, where they have choked 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. Al- 

 though 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 show- 

 ering 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 Giants' 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 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 intense heat of 

 melted matter. 



The absence of cones and craters, and long narrow streams of super- 

 ficial lava, in England and many other countries, is principally to be 

 attributed to the eruptions having been submarine, just as a considerable 

 proportion of volcanoes in our own times burst out beneath the sea. 

 But this question must be enlarged upon more fully in the chapters on 

 Igneous Rocks, in which it will also be shown, that as different sedi- 

 mentary formations, containing each their characteristic fossils, have 

 been deposited at successive periods, so also volcanic sand and scoriae 



