16 



KNOWLEDGE. 



[January 1, 1897. 



formation and structure of volcanoes are by no means 

 obscure. Von Bucb, Humboldt, and others held the 

 opinion that they have been formed by upheaval, and 

 have their upper portions only composed of ejected matter. 

 It is now, however, conclusively established by observation 

 of sections of cones, and especially by the formation of 

 Monte Xuovo, near Baice, which was actually observed in 

 the year 1588, that a volcano is simply an accumulation 

 of ejectamenta around a central orifice. All the active 

 volcanoes of the world are, therefore, mountains in course 

 of formation at the present time, and, so to speak, not yet 

 completed. Many of them, too, were only commenced in 

 very late geological time. The best-known of all volcanoes, 

 Tesunus and Etna, may be taken as types of the present 

 day active volcanoes of the world. Vesuvius, including 

 the old crater ridge of Monte Somma and the gigantic 

 cone of Sicily, had a late Pliocene beginning. 



One very interesting fact in connection with the 

 formation of mountains by volcanic action is that such 

 action sometimes destroys as well as builds up, and often 

 lowers the height of a volcano. This is strikingly exem- 

 plified in the history of Vesuvius, for in prehistoric times 

 this mountain was, there is little reason to doubt, at least 

 double its present elevation, and in historical and even in 

 recent times the present cone has been greatly reduced 

 in height by unusually great or paroxysmal eruptions. 

 Small eruptions increase the elevation of a volcanic cone, 

 whOe great eruptions lessen it by blowing ofi" its upper- 

 most part and so increasing the size of the crater, as is 

 shown by the accompanying illustrations.* But perhaps 

 the most conspicuous illustration of the destroying action 

 of volcanic energy was that afforded only about twelve 

 years ago at the island volcano, Krakatoa, in the Sunda 

 Straits, an interesting account of which terrific eruption 

 appeared in Knowledge for December, 1895. 



The volcanic mountains next in age are those which 

 have been quiescent for many centuries and so are thought 

 to be extinct, but yet have been in eruption during the 

 historic period. Monte Epomeo, in the Island of Ischia, 

 in eruption in 1302, is an instance of this class, as is also 

 the lofty mountain in the North-West of Arabia, con- 

 sidered by some to be the true Mount Sinai, and which is 



Flo. 2.— Cone Truncated bv C4reat Eruptions. 



stated by the Arabs to have been in eruption in ancient 

 times. The singularly regular and beautiful cone of Fusi 

 Yami, in Japan, is, too, an excellent example, as is also the 

 Solfatara of the classic Phlegrean Fields. 



The many mountains that have been built up by volcanic 

 action since Pliocene times, and have yet never been active 

 since the commencement of the historic period, constitute 

 a still older class. Nowhere furnishes better examples of 

 the prehistoric volcanoes than Auvergne in Central 

 France, where volcanic cones and craters are exceedingly 

 numerous, and no less than forty cluster around the Puy 

 de Dome and form the beautiful district near Clermont. 

 These have been made extensively known by the admirable 

 and profusely illustrated work of Poulett Scrope, " The 

 Volcanoes of Central France." 



* From "Moimt. Vesuvius," by J. L. Lobley. 



Although the Puys of Auvergne are undoubtedly pre- 

 historic as active volcanoes, yet some of their craters are 

 so perfect that they cannot belong to a period more 

 remote than that immediately preceding the present epoch. 

 That the greater portion of the mass of these mountains 

 is of post-Pliocene age is obvious from their lavas covering 

 Pliocene strata, though the volcanic action here com- 

 menced, probably, in that period of great volcanic dis- 

 turbance, the Miocene. Doubtless some ceased their 

 activity long before others, as the craters of a few of the 

 Puys are much worn and wasted. To the same era may 

 be assigned the great volcanic mountain of Cantal, and 

 the other extinct cones of Central France. 



Of this general age, too, are the beautiful and interesting 

 volcanic hills in the district of the Eifel, near the Ehine, 

 the crater of one of which forms the Lake Maare. Better 

 known are the extinct volcanoes near Eome, displaying 

 fine examples of lakes of placid water occupying craters 

 once the scene of violent volcanic action. Lago di 

 Bolsena, occupying one of the largest craters in the world. 



Fig. 3. —Vesuvius after Eruption of 1G31. 



Lago di Braceiano, and the well-known Lake of Albauo, 

 are perfect examples. From the volcanoes of the Alban 

 Hills the material was ejected that now forms the tufa 

 constituting the " Seven Hills " of Eome. The famous 

 Lake Avernus and the Lago Agnano are lakes in craters 

 of this age near Naples; and the hills, and indeed the whole 

 site of that great city, are formed of the ejectamenta of 

 those and other neighbouring craters. 



The great volcanic mountain of Tatra in the North of 

 Hungary, and other volcanic elevations on the flanks of 

 the Carpathians and in Transylvania, may be ascribed to 

 the earlier Oligocene period, and the high basaltic lands of 

 South AustraUa and Victoria to Eocene volcanic action. 



One of the greatest outpourings of lava the world has 

 seen occurred in Cretaceous times, and formed the high 

 plateau of the Deecan in India, where volcanic rocks, 

 covering an area of two hundred thousand square miles, 

 have a maximum thickness of no less than six thousand 

 feet. To this period also belong vast uplands of volcanic 

 rocks in the West of the North American continent. 



Though in the older geological periods volcanic activity 

 gave vast thicknesses of rocks now occupying elevated 

 positions, and sometimes forming hills and even the sum- 

 mits or parts of the summits of lofty mountains, yet the 





Fig. I. —Vesuvius as at Present. 



hills and mountains of which they now form portions were 

 elevated in most cases subsequently to the outpouring of 

 the lava, since it is interbedded with sedimentary rocks. 

 Conspicuous examples of this are seen in the Welsh 



