CAUSE OF VOLCANOES. 125 



basaltic varieties contain a considerable proportion of iron, 

 the feldspar being 'replaced by other minerals, as leucite, 

 they have a dark ferruginous aspect. Mr. Scrope adds a 

 third class, which from being neither decidedly trachytic, nor 

 basaltic, from frequently containing both iron and feldspar, 

 and from being of a tint between the two, he denominates 

 grey-stones. The other volcanic productions are obsidian, or 

 volcanic glass, produced by a rapid cooling ; pumice, which 

 is volcanic froth, produced by the access of vapour to the 

 mass when in a fluid condition; and aqueous lava or 

 volcanic mud, caused by the admixture of torrents of rain, 

 or of melted snows, which accompany these eruptions, and 

 which, as they consolidate, form rocks of earthy aspect, 

 called tuff" or tufa. These substances frequently pass into 

 each other, both pumice and tufa constantly exhibiting transi- 

 tions into obsidian. As a general idea of the products of 

 active volcanoes, it may be conceived that all lava is essen- 

 tially composed of the same elementary fluid substances, and 

 that its subsequent character and appearance are occasioned 

 by the different agencies to which it is subjected. If cooled 

 rapidly it becomes a glass ; if slowly, a rock, more or less 

 crystalline, in proportion as its rate of refrigeration is more 

 or less quickened ; while the presence of gas and its extrica- 

 tion in cooling, leaves it a light and cellular mass.* 



CAUSE OF VOLCANOES. The primary cause of these phe- 

 nomena is among those problems, the solution of which 

 is reserved for a more advanced state of knowledge. The 

 great quantity of gaseous vapour evolved during eruptions as 

 well as the general proximity of volcanoes to the sea, have 

 led to the supposition that water is the active agent in pro- 

 ducing these phenomena, and the two hypotheses proposed to 

 account for their origin, have reference to aqueous action, as 

 having a considerable share in their production. The first 

 ascribes volcanic eruptions and earthquakes to the effects 

 produced on a heated nucleus, mechanically disturbed by the 

 access of water ; the other explains the phenomena by sup- 

 posing the decomposition of water to be effected by means 

 of alkaline metals, existing in the interior in an uncombined 



* See an interesting chapter on this subject in Humboldt's Views of Nature 

 Bonn's edition, p. 353. 



