DYNAMICAL GEOLOGY. 257 



v Von Buch allowed, have flowed into their present position, 

 but not superficially like the lava streams of an active volcano, 

 only below the surface and under great pressure. The more 

 important points in Von Buch's chain of evidence were the 

 occurrence of coarse-grained crystalline rocks in the bottom 

 of the Palma cauldron, the general arrangement of the strata 

 sloping away from the central crater and penetrated by numer- 

 ous dykes, and the presence of deep ravines (Barrancos), 

 which he regarded as eruptive fissures on the outer side of 

 the crater of elevation. Von Buch thought craters of elevation 

 were very numerously distributed ; some of them originally 

 embracing a central volcanic cone, for example, the island of 

 Bourbon ; others, such as those of Auvergne, the Siebenge- 

 birge near Bonn, the Lipari Isles, Etna and the American 

 Cordilleras, being trachytic dome-shaped mountains situated 

 above the fissures of elevation, and either remaining intact at 

 their summit or providing themselves with orifices of ejection. 



Von Buch sub-divided all the volcanoes on the earth's sur- 

 face into two classes central and serial. The former, accord- 

 ing to Von Buch, are located centrally with reference to a 

 large number of outbreaks radiating in all directions; the 

 latter mark the position of long crust-fissures, and either form 

 the highest ridge of a terrestrial mountain-system, or if the 

 volcanic fissure be submarine, the highest summits emerge as 

 islands above the ocean. 



While Von Buch in his theory tacitly accepted Hutton's 

 principle, that the upheaval of the solid rocks was due to the 

 expansive force of subterranean heat, he re-cast this doctrine 

 into the particular form required to explain his own con- 

 ceptions of volcanicity. He formed the erroneous idea that 

 the inclination of the basalts around a volcanic vent could 

 only be due directly or indirectly to crust-elevation, and this 

 view shipwrecked a theory which otherwise embodied some 

 valuable generalisations. Adapting his theory to the termin- 

 ology of the present day, Von Buch's conception of a "Central 

 elevation-crater " represented a local exhibition of crust-expan- 

 sion accompanied by a local inrush of molten and gaseous 

 material towards a centre of crust-weakness, and the escape of 

 the same at a central vent; Von Buch's "Serial elevation- 

 craters" represented the results of a regional exhibition of 

 the expansive forces due to internal heat, and regional admis- 

 sion of molten rock and gaseous vapours into zones and areas 



17 



