PART I. CHAPTER VII. 91 



Cones and Craters. 



alumina, lime, potash, iron, and the rest, are in proportions well 

 fitted for vegetation. As they do not effervesce with acids, a 

 deficiency of calcareous matter might at first have been appre- 

 hended ; but although carbonate of lime is rare, except in the 

 nodules of amygdaloids, yet it will be seen that lime sometimes 

 enters largely in the composition of augite and hornblende. (See 

 Table, p. 102.) 



In regions where the eruption of volcanic matter has taken 

 place in the open air, and where the surface has never since been 

 subjected to great aqueous denudation, cones and craters are 

 strikingly characteristic. Many hundreds of these cones are 

 seen in central France, in the ancient provinces of Auvergne, 

 Velay, and Vivarais, where they observe, for the most part, a 

 linear arrangement, and form chains of hills. Although none 



Fig. 86. 



Part of the chain of extinct volcanos called theMonts Dome, Jluvergne. 

 (Scrope.) 



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 exist- 

 ing valleys. The origin of the cone and crater-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. Al- 

 though extremely heavy, this lava is forced up by the expansive 

 power of entangled gaseous fluids, chiefly steam or aqueous va- 

 pour, exactly in the same manner as water is made to boil over 

 the edge of a vessel when steam has been generated at the bot- 

 tom 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 



