594 



VOLCANIC KOCKS. 



[Ch. XXVIII. 



constitute the most striking peculiarity of this class of formations. 

 Many hundreds of these cones are seen in central France, in the an- 

 cient provinces of Auvergne, Yelay, and Vivarais, where they observe, 

 for the most part, a linear arrangement, and form chains of hills. 

 Although none of the eruptions have happened within the historical 

 era, the streams of lava may still be traced distinctly descending from 

 many of the craters, and following the lowest levels of the existing 

 valleys. The origin of the cone and crater-shaped hill is well under- 



Eig. 6T4. 



Part of the chain of extinct volcanoes called the Monts Dome, Auvergne. (Scrope.) 



stood, the growth of many having been watched during volcanic erup- 

 tions. A chasm or fissure first opens in the earth, from which great 

 volumes of steam and other gases are evolved. The explosions are so 

 violent as to hurl up into the air fragments of broken stone, parts of 

 which are shivered into minute atoms. At the same time melted 

 stone or lava usually ascends through the chimney or vent by which 

 the gases make their escape. Although extremely heavy, this lava is 

 forced up by the expansive power of entangled gaseous fluids, chiefly 

 steam or aqueous vapor, exactly in the same manner as water is made 

 to boil over the edge of a vessel when steam has been generated at 

 the bottom by heat. Large quantities of the lava are also shot up 

 into the air, where it separates into fragments, and acquires a spongy 

 texture by the sudden enlargement of the included gases, and thus 

 forms scoria, other portions being reduced to an impalpable powder 

 or dust. The showering down of the various ejected materials round 

 the orifice of eruption gives rise to a conical mound, in which the 

 successive envelopes of sand and scoriae form layers, dipping on all 

 sides from a central axis. In the mean time a hollow, called a crater, 

 has been kept open in the middle of the mound by the continued 

 passage upwards of steam and other gaseous fluids. The lava some- 

 times flows over the edge of the crater, and thus thickens and 

 strengthens the sides of the cone ; but sometimes it breaks down the 

 cone on one side (see fig. 674), and often flows out from a fissure at 

 the base of the hill, or at some distance from its base.'* 



Composition and Nomenclature. — Before speaking of the connection 

 between the products of modern volcanoes and the rocks usually styled 

 trappean, and before describing the external forms of both, and the 



* For a description and theory of active volcanoes, see Principles of Geology, 

 chaps, xxiv. et seq. and xxxii. 



