462 



COMPOSITION AND NOMENCLATURE [Ch. XXVIII. 



Auvergne, Velay, 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 



Fig. 621. 



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



cone and ciater-shaped hill is well understood, the growth of many having 

 been watched during volcanic eruptions. 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 scoriw y 

 other portions being reduced to an impalpable powder or dust. The 

 showering down of the various ejected materials round the orifice of erup- 

 tion 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 sometimes flows over the edge of the 

 crater, and thus thickens and strengthens the sides of the cone ; but some- 

 times it breaks down the cone on one side (see fig. 621), and often it 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 

 manner and position in which they occur in the earth's crust, it will 

 be desirable to treat of their mineral composition and names. The 

 varieties most frequently spoken of are basalt and trachyte, to which 



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

 chaps, xxiv. et $eq. and xxxii. 



