522 Revieus — A. Sfiibel — Diversity in Eniptive Mountains. 



is unfortunate that it was not defined more precisely ; thus it would 

 have been preferable, in every way, merely to have pointed out that, 

 during Tertiary times, volcanoes established themselves in many 

 regions where previously, for long ages, there had been no trace of 

 igneous activity. The pith of the argument, developed from this 

 text, is that the initiation of a volcanic regime must be a very dilferent 

 affair from its continuation. Lyell's philosophy is discarded, and in 

 its place we are asked to accept speculation of a kind not easy to test 

 by direct observation. 



It is claimed that the volume of magma emitted as lava or 

 agglomerate during any particular eruption must be proportional to 

 the resistance overcome during the eruption. The actual initiation 

 of a volcano is therefore accompanied by a very extensive extrusion 

 of magma. In fact Stiibel regarded it as logically certain that 

 volcanoes are in the main monogene structures, each resulting fi'oni 

 the eruption which gave it its birth. That monogene volcanoes do 

 exist is of course well known, since Monte j^uovo and other historical 

 examples may be cited ; the novelty of Stiibel's position resides in 

 his belief that volcanoes are all of them essentially monogene, though 

 in certain cases their structure may be considerably modified and 

 obscured by later polygene accretions. 



Topographical considerations are exalted to the position of prime 

 importance in deciphering the life-history of volcanoes. Thus an 

 acute cone with a relatively small crater is regarded as the only form 

 attainable by a polygene volcano. A flattened dome or cone on the 

 other hand indicates a monogene structure. Such a dome or cone may 

 arise either through a process akin to bubble-blowing, producing 

 a mass with a liquid interior enclosed in a solid cracking crust, or 

 else through the rapid superposition of lava-flow upon lava-flow, with 

 or without intercalations of agglomerate. Monogene volcanoes built 

 on this latter plan are supposed to have a comparatively flat strati- 

 fication, and to owe their conical form merely to a gTadual exhaustion 

 of the subterranean reservoir in each individual case, and to the 

 consequent curtailment of successive outpourings of lava. An 

 interesting suggestion places the origin of the parasitic cones of Etna 

 in the body of the volcano, and not in its almost exhausted deep- 

 seated reservoir. 



A monogene cone, arising in the manner indicated above, may not 

 have a crater at all, but on the other hand it may have one of 

 relativelj' immense size. An eruption having spent its force, and 

 nearly or quite emptied its limited magma-basin, is frequently 

 brought to a close by an episode of rapid retreat, and the unconsoli- 

 dated material of the central part of the cone flows or tumbles back 

 into the i)ipe and so leaves behind a gaping caldera. This supposed 

 connexion between calderas and tlie monogene origin of volcanoes is 

 the point most strongly emphasized by the author. His conclusions 

 are supported by an appeal to the moon, where the caldera form 

 of crater is typically developed. The regular recurrence of the 

 phenomenon in this case does seem to favour some simple and uniform 

 explanation such as is afforded by the theorj- of the monogene eruption ; 

 but one cannot forget the uncertainty as to whether lunar craters 



