sur la Theorie des Volcans. ^f 



A singular fact, before pointed out by other writers, is next 

 commented on ; viz., that all volcanoes to the north of the 

 equator have all their orifices to the west, and constantly 

 discharge their lavas to the south; while those to the south of 

 the equator as invariably follow an opposite direction, (p. 35.) 

 Strombolo being an exception, the author accounts for it by 

 assuming that the great cataclysm removed the original crater 

 and made a breach lower down in the N.N.W. side. The 

 action of the ejected materials is said to be spiral in the 

 crater, and the parabolas described by the projectiles to be 

 from an axis turned to the south, (p. 37.) Tlie exciting 

 cause of volcanic action is attributed to sea water finding 

 its way to the fermenting matter, producing increased heat 

 and dilatation of gases ; the change being proportioned to the 

 calibre of the volcano, and when it exceeds this destroying 

 the volcano, (p. 37.) This spiral motion of the materials 

 in action may, perhaps, be borne out by the circular motion 

 in earthquakes ; but M. de Bylandt does not allude to it. A 

 second cause of excitement is attributed to atmospheric air 

 rushing down so as to produce regular respirations. The deto- 

 nations are refen'ed to the noise occasioned by the separation 

 of the molecules in the rocky masses acted on, and the evolu- 

 tion of hydrogen gas, combined with electric shocks, (p. 38.) 

 The flow of lava is said to be uniform, and to follow the direc- 

 tion of the axis of the crater ; and examples are quoted where 

 lava streams have been diverted from their direction, by 

 walls at an angle ; as in the case of the eruption of 1 669, 

 when the garden wall of the Benedictine convent saved the 

 city of Catania. In 1831-32, the author also tried the 

 experiment, and says that it is possible, by taking advantage 

 of the slowness of the current, to prevent its ravages, (p. 39.) 

 We have no space for the calculations of the author on the 

 heights and forces of volcanoes : but we may mention that 

 he divides lavas, as follows, into seven classes : — 1. Lithoid ; 

 2. Compact ; 3. Trachites ; 4>. Basaltic ; 5. Obsidian or vi- 

 treous ; 6. Scoriae ; 7. Pumice. He, moreover, adds, that 

 the materials differ according to different situations and dif- 

 ferent epochs of each. (p. 42.) 



Such are the principal subjects to be discussed in M. de 

 Bylandt's work, of which the ^^ avant-propos" gives an out- 

 line. The first volume is to contain an introduction in con- 

 nection with La Place's theory of the world, ending with 

 an account of the causes and effects of the currents o^ the 

 sea and the trade winds, and Captain Franklin's tables of 

 magnetic phenomena, (p. 43.) The second volume is to em- 

 brace the effects of water in motion, the parallel betw««n the 



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