VULCANOLOGY AND SEISMOLOGY. 477 



On the other hand, paroxysmal eruptions often begin with a violent 

 burst, followed by u series of explosions, with but little extrusion of lava, 

 and sometimes with absolutely none at all, only scoriae being emitted, 

 as at Cosaguina and Krakatoa. The author would account for the pres- 

 ence of great quantities of water in some eruptions by the admission of 

 the surface waters, or sometimes of sea water, to the volcani(; duct dur- 

 ing the eruption itself, and gives diagrams of the altered water levels 

 about a volcano during an eruption to show the possibility of such ad- 

 missions. Concluding, then, that water plays only a secondary part in 

 originating volcanic plienomenn, he is inclined to recur to tlie old hy- 

 pothesis of a comparatively thin, flexible earth-crust resting upon a 

 molten substratum; but instead of adopting anew the theory of a fluid 

 interior to the earth, which physical reasoning has shown to be untenable, 

 lie thinks the phenomena are best accounted for by the supposition of a 

 thin crust, resting on a viscid layer also of no great thickness, and wrap- 

 ping round a solid nucleus. He concludes that the viscid magma thus 

 compressed between two solid layers by the secular contraction of the 

 globe is extruded where it may find or make an opening. And while 

 the extravasation of lava is due to these causes, the presence of vapor 

 is due only to the surface and underground waters with whicli the lava 

 comes in contact as it rises through the volcanic duct. [Nature^ xxxi : 

 592.) 



Siuiilar opinions as to the part played by water in volcanic outbursts, 

 and the source from which the water is derived, were expressed before 

 the Geological Society by H. J. Johnston-Lavis. {Nature, xxxii: 69.) 



The discussion of jthe theoretical form of volcanic cones, begun by Pro- 

 fessor Milne in 1878 and 1879 {GeoL Mag.), is renewed and carried on by 

 (reorge F. Becker. {Am. Jour. ScL, xxx : 283.) Milne had regarded the 

 problem as being that of the form assumed by loose materials, cinders, 

 ashes, etc., and concludes that it would be that generated by the revolu- 

 tion of a logarithmic curve round its asymi)tote. But Becker regards the 

 mountain as essentially a continuous mass, and deduces the equation of 

 the curve, which by its revolution will generate the finite unloaded column 



of " least variable resistance," for which he finds ^ = - — "T"^*" • This 



c 2 



is a special case of the equation ^, _ j^ ^-^ -f B f "^ which he has elsewhere 

 shown to characterize the arrangement of sheets of rock in a complex 

 fault, and which belongs also to several other physical pheuomeuii. On 

 comparing graphically this theoretical curve with actual outlines drawn 

 from photographs of volcanic cones, as with Fusiyama and Kumagatake 

 in Japan, and Hood and Popocatapetl in America, the similarity of the 

 several outlines is at once evident. 



Velain's book "Les Volcans" is briefly reviewed by Captain Duttoii, 

 stating the contents of the several chapters, with comments thereon. 

 {Science, vi : 255.) 



