DYNAMICAL GEOLOGY. 26 1 



form of the volcano is determined by several causes, for 

 example, the inequality of the ground, violent winds during 

 eruption, or any obstacles within the vent which may impede 

 the ascent of the lava, or direct it into another course. 

 Stratification is apparent in the structure of the cone of 

 ejection ; it is especially clear when there is an alternation of 

 lava and volcanic ash. The inclination of the layers of 

 volcanic rock is always from the edge of the crater to the base 

 of the cone. The liquidity of the lava depends on its 

 composition, texture, and temperature, and according to these 

 and to the superficial relations, the solidified lava assumes the 

 form of horizontal sheets, thick masses, or dome-shaped cones. 

 During the cooling of the lava the escape of the vapours gives 

 origin to the slaggy, vesicular structure of the lava; the 

 liberation of the gases from the lava produces all kinds of 

 minerals, and may take place either in association with 

 escaping vapours as " fumaroles," or independently as gaseous 

 emanations or " solfataras " ; sometimes the gases collect from 

 hot springs, or they vanish as exhalations. Pillar-shaped, 

 rounded, cubical, rhomboidal, flaggy or shaly structure 

 develops in consequence of the contraction of the lava 

 during the processes of cooling. 



As one and the same volcano may emit basaltic and trachytic 

 lavas, Scrope thought it probable that all volcanic products 

 come from the same subterranean magma, and that their 

 specific difference is due to some condition connected with 

 the access of heat and the subsequent chemical processes 

 during their ascent. Poulett-Scrope opposed the conception 

 of Humboldt and Buch, that trachyte and basalt rocks are of 

 different ages. The larger volcanic mountains, he said, clearly 

 owe their origin and form to repeated eruptions ; the original 

 cones of ejection are rent by later outbreaks, and the repeated 

 outpourings and injections of lava still help to strengthen 

 the mountain. In the summit crater, for the most part only 

 vapours escape, together with the blocks and fragments which 

 are carried up by the explosions. Very wide and deep craters 

 form during the violent paroxysms of a volcano ; by means of 

 the subsequent eruptions new cones of ejection may arise 

 within this deep crater, and be surrounded by the circular 

 wall of the old crater ; or the wall of the old crater may be 

 disturbed and partially destroyed by a new crater (Somma). 



Scrope strongly contested the existence of craters of elevation, 



