1886.] NEW YORK ACADEMY OF SCIENCES. 2& 



quakes, and the flexibility of the earth's crust with a solid in- 

 terior. Some have gone back to Sir Humphry Davy's theory, 

 that volcanoes were the product of intense chemical action in 

 certain circumscribed portions of the earth's mass; and others 

 have supposed that, between a thick external crust and a solid 

 interior there was an intermediate zone of fused matter from 

 which volcanic ejections emanated. Mallet, who has displayed 

 great learning and ability in his various papers on volcanoes and 

 earthquakes, forbidden by Sir William Thomson^s dictum from 

 drawing molten matter from the interior of the earth to operate 

 his volcanoes, contrived a method of manufacturing it on the 

 spot. He proposed a theory that all the phenomena of vulcanism 

 are due to thearchingof the exterior strata composing the earth's 

 crust, their final yielding to gravity and crushing down on to the 

 contracting interior; the conversion of motion into heat produc- 

 ing all the thermal phenomena. 



A fatal defect in this theory is that it gives no reason for the 

 localization of the heat along the line of fissure from which the 

 lava flows. All parts of the masses on either side must share in 

 the motion and should also share in the heat, and we must look 

 elsewhere for an explanation of the phenomena.* 



There would have been no question of the truth of the old 

 theory of vulcanism if it had not been raised by the physicists 

 whose names have been mentioned, and it can now be seen that 

 their objections have little force. Delaunay, of Paris, and Hen- 

 nessy, of Dublin, have shown that the premises assumed by 

 Thomson, Hopkins, and Pratt in their attempted refutation of 

 the old theory of a comparatively thin crust are not those of 

 nature, and that their conclusions are, as a consequence, irrel- 

 evant and valueless. Their objections were aimed at an incom- 

 pressible fluid interior and an elastic crust ; conditions which do 

 not and could not exist. Beside this there must be a viscous 

 zone of considerable thickness in which the transition from the 

 solid crust to the liquid interior is very gradual ; and it is highly 

 probable that the matter of this viscous zone is not only not it- 



^ In a review of Mr. Clarence King's report " On the Geology of the 

 Country Bordering the Fortieth Parallel," the writer in 1879 suggested 

 a simple explanation of the phenomena of vulcanism, viz., A slight 

 arching of the crust of the earth along lines of fracture and elevation, 

 in a measure relieves the pressure by which highly heated matter below 

 is kept in coerced solidity. This relief of pressure causes the potential 

 fluidity of this compressed matter to become actual, and thus reservoirs 

 of lava are called into existence beneath the lines of fracture and arch- 

 ing. Finally the jjressure from gravity being maximum under the 

 tables of unbi'oken strata on either side, and minimum beneath the 

 crown of the arches, this unequal pressure causes the lava to rise along 

 the fisi-ures and flow out in volcanic eruptions. 



