256 HISTORY OF GEOLOGY AND PALAEONTOLOGY 



the Canary Islands is full of information for the geographer, 

 meteorologist, botanist, and geologist. The chapter on the 

 geological relations is a model of skilful and methodical 

 exposition. The form, the structure, the composition and 

 origin of the different islands, the constitution of the rocks 

 and volcanic ejecta, are depicted in a manner at once 

 attractive and scientific, and the context is illustrated by 

 topographical maps of Teneriffe, Palma, and Lancerote, 

 prepared exclusively from surveys and drawings made by 

 Von Buch. At the Peak of Teneriffe and in the wonderful 

 basin-shaped depressions (" Calderen ") in Palma and Canaria, 

 Von Buch found new evidences of volcanic elevations. And 

 from this time forward the "Elevation Crater" became one of 

 his pet theories. 



The first public enunciation of the theory was given by Von 

 Buch on the 28th May, 1819, in the Berlin Academy. He 

 defined true volcanoes as solitary, conical mountains almost 

 always composed of trap-porphyry (trachyte), and from which 

 fire, vapour, and stone are emitted. . They are surrounded by 

 molten rock or ashy material which flows downward in the 

 form of streams. Typical volcanoes are distinguished by Von 

 Buch's theory from larger basaltic masses which after emission 

 have been uplifted around the areas of volcanicity. These vol- 

 canic uplifts were said to be characterised by the absence of 

 lava streams or of accumulations of rapilli round a central 

 area, and likewise by the predominance of basaltic over 

 trachytic rocks. The basaltic masses are inclined similarly to 

 sedimentary strata in any upheaved area ascending from every 

 side towards a great central cauldron, or crater of elevation. 

 This crater might be afterwards closed by the collapse of the 

 upheaved rocks, and might be opened intermittently by fresh 

 volcanic ebullitions from below. 



Von Buch then argued that the force required to create 

 such a crust-disturbance must be enormous, and must repre- 

 sent the prolonged accumulation of a store of energy in the 

 earth's interior. The expansive force of the heated lava first 

 bulging the rocks upward like a blister or dome, might go on 

 increasing until it rent them asunder and provided an outlet 

 for the ascending vapours. No true volcano formed unless, 

 as frequently happened, a central cone of ejected material 

 gathered within the crater of elevation. 



The upper basaltic layers of the crater of elevation might, 



