114 Scientific Proceedings, Royal Dublin Society. 



Source of Energy stored up in Igneous Matter. — Disregarding for 

 the time being the unsettled question of the condition of the earth's 

 interior, let us assume 1 that we have an almost unlimited supply 

 of igneous material ; let us then ask ourselves, whether this material 

 supplied in varying proportions is capable of producing an equiva- 

 lent difference in the display and character of the volcanic forces. 

 A moment's consideration will satisfy us that such is certainly not 

 the case. There is no doubt that in a great ex-plosive eruption a 

 very large amount of matter may be ejected, but as it is always in 

 the form of pumice its apparent bulk is larger than its real one. 

 On the other hand, in paroxysmal eruptions we have enormous 

 quantities of igneous magma ejected in a fluent form without 

 exhibiting that amount of energy that occurs in the first case. If 

 we compare the amounts, by weight, of matter {essential) 2 ejected 

 during the Plinian eruption of Vesuvius with the lava outpours of 

 1631, we should find no ratio between them and the eruptive energy 

 exerted on each occasion. Many still more striking examples 

 might be given of the gigantic though comparatively quiet outflows 

 of basalt when compared with low crater rings of the Eifel and 

 other volcanic districts. 



Were facts otherwise, so that the greater the eruption the 

 greater the amount of material extruded, we should then have 

 fairly conclusive evidence that the water which is the main motive 

 power in a volcano was contained uniformly diffused throughout 

 the igneous magma, as held by Rev. 0. Fisher and others. 3 Of 

 course we must admit that in distant regions such might really be 

 the case, but it is not reasonable to suppose that it is so in a single 

 locality, a necessary datum for such an argument. In consequence 

 of this we are reduced to search for some local influences that are 

 brought to bear upon an isolated portion of the igneous magma, 

 and the only rational way in which we can suppose any such mass 

 to be isolated would be when it has entered its duct on its way 

 from its source to the surface. 



1 This is compatible with all the more reasonable theories regarding the physical 

 state of the deeper parts of our globe. 



2 I mean only that which is really extruded de novo, and not materials torn from the 

 crater sides. 



3 Physics of the Earth's Crust, 1881, p. 187. 



