

ON ITS EXTERIOR. VOLCANOES. 225 



slope of the upheaved strata. A mine-like explosion, 

 the bursting of the central and highest portion of the 

 convex swelling of the ground, produces sometimes only 

 what Leopold von Buch has termed a " crater of eleva- 

 tion ;" ( 308 ) i.e. a crater-shaped, round or oval, depression 

 bounded by an " elevation-circus," a ring-shaped, and 

 in most cases partly broken down, rampart or encircling 

 ridge, and sometimes also at the same time when 

 the structure of a permanent volcano is to be completed 

 a dome-shaped or conical mount in the middle of 

 the crater of elevation. This inner mount is, in the 

 greater part of such cases, open at its summit, and on 

 the floor of this opening (or the floor of the crater of 

 the permanent volcano), there rise non-permanent hills 

 of erupted substances and scoriae, small and great 

 " cones of eruption ; " in Vesuvius these sometimes rise 

 much higher than the crater-walls of the cone of eleva- 

 tion. It is not always, however, that all the witnesses 

 to the first eruption all the different parts of the old 

 framework or scaffolding, such as I have here described, 

 are preserved. In many of the greatest and most active 

 volcanoes the high escarpment forming the periphery of 

 the crater of elevation cannot be traced even in frag- 

 ments or ruins. 



A great service has been rendered in modern times, 

 not only by the careful comparison of volcanoes situated 

 at remote distances from each other, and by the more 

 exact examination of their several relations of form ; but 

 also by the introduction of more definite expressions, 

 whereby significant differences in relief, as well as in the 

 manifestations of volcanic activity, are distinguished 

 apart. If one is not decidedly averse to all classification, 



VOL. iv. Q 



