ORIGIN AND STRUCTURE OF VOLCANIC CONES. 



^ 



opposite view, whicli will be immediately explained, claimed as their 

 adherents Sir Charles Lyell, Poulett Scrope, and others. 



In Figs. 3 and 4 are diagrammatic representations of the two the- 

 ories. 



The upheaval] sts believed that the earth-crust actually surrounding 

 the vent was bodily lifted up by the subterranean igneous forces into 

 a dome-shaped or bubble-like mass, thus forming the main mass of the 



Fig. 4. Crateb of Upheaval. 



cone, of which the center was the point of fracture, and therefore the 

 vent. The ejecta were therefore considered to form only a thin super- 

 ficial crust covering this. The subjacent rock which had been elevated 

 would thus have a quaquaversal or periclinal dip away on all sides 

 from the chimney (Fig. 4). 



The opponents to this view attribute the entire bulk of the moun- 

 tain to the ejecta, as seen in Fig. 3, the only change in the basement 

 beds being those produced by pressure and excavation, both of which 

 tend to make them dip toward the vent, thus producing quite a con- 

 verse effect to the former. 



This latter view certainly seems the most feasible, and, after a care- 

 ful examination of many of the old craters broiiQ:ht forward bv the 

 upheavalists as evidence, one becomes satisfied that they have wroDgly 

 interpreted facts, which the more advanced state of knowledge at the 

 present day and the collected experience of subsequent observers make 

 easy to our perception. On the other hand, it would be undoubtedly 

 rash to conclude that all craters were foimed entirely on one or the 

 other model. JoruUo, in Mexico, for instance, has many points about 

 it to support the upheaval theory. David Forbes, that clear observer, 

 mentions many facts about South American volcanoes that should 

 deter us from admitting the formation of cones and craters by the de- 

 position of ejecta only. 



The rapidity with which a volcanic cone may be raised is a point 

 of great interest. We hear every now and then of some small island 

 appearing and again disappearing below the sea almost as rapidly as 

 it rose. Probably, however, the best illustration is that of Monte 

 Xuovo, four hundred and fifty-six feet high, situated in the Campi 

 Phlegraci, about eight miles west of Naples. This was raised from a 



