PART I. CHAPTER I. 



19 



Volcanic Origin of certain Rocks. 



leaving only the harder and more solid materials. By this ero- 

 sion, and by earthquakes, their internal structure has occasion- 

 ally 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 scoriee, but also perpendicular 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 showering down from the air, 

 or incumbent waters, of sand and cinders, first shot up from the 

 interior of the earth by 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 volcanos may be seeji with cones, craters, and often 

 accompanying lava-streams. 



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 columjiar structure and mineral composition with 

 streams of lava which we know to have flowed from the craters 

 of volcanos. We find also similar basaltic 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 lava, in England and elsewhere, is principally attributed by 

 geologists to the eruptions having been formerly submarine, just 

 as a considerable proportion of volcanos 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 sedimentary formations, contain- 

 ing each their characteristic fossils, have been deposited at suc- 

 cessive periods, so also volcanic sand and scorise have been 

 thrown out, and lavas have flowed over the land or bed of the 

 sea, at many different epochs, or have been injected into fissures ; 

 so that the igneous as well as the aqueous rocks may be classed 

 as a chronological series of monuments, throwing light on a 

 succession of events in the history of the earth. 



