128 Scientific Proceedings, Royal Dublin Society. 



been apparently extinct, and that in the great Plinian eruption we 

 had nothing hut spongy fragmentary varieties of its usual igneous 

 rock. The eruptions that followed the Plinian one occurred at 

 diminished intervals, and so the more did their products approach 

 in structure that of the lava of chronic activity, until, in the tenth 

 century, pumiceous materials formed no longer, at any rate as far 

 as the essential ejectamenta go, the products of these eruptions. 

 To take another example, the precedents and whole history of 

 which is pretty well known, namely, Monte Nuovo. We find 

 that the main mass of the mountain is built up of pumice in 

 various stages of comminution ; capped, or covered, by more 

 compact and crystalline scoria, or lava, fragments, which were 

 only ejected at the last, when the volcano tended to pass into the 

 chronic condition. We also know that such volcanoes as Tomboro, 

 Krakatoa, and others like them, after a long quiescence burst 

 forth with an amount of violence to cause disturbances throughout 

 our planet, produce ejectamenta that are always of pumiceous 

 character. 



These facts, I think, give us the clue to the real sequence of 

 phenomena whioh lead up to, and result in, the different varieties 

 of eruption. Let us suppose that an extensive igneous dyke has, 

 from some circumstances which will be discussed later on, become 

 plugged at the exit on the earth's surface. The part of the 

 magma that retains a sufficiently high temperature will be 

 gradually absorbing water ; and as assimilation proceeds the 

 tension of the magma will proportionally increase, and the tem- 

 perature of the mixture reduced, so that in some cases this loss 

 will gradually favour solidification of portion of the magma 

 forming the crystals of the felspars, Amphibole, Biotite, and 

 other micas, &c. Such a state of things will go on until one 

 of two things takes place — either the loss of heat be such that 

 the whole igneous mass solidifies, or, on the other hand, the 

 tension overcomes the resistance, and an eruption takes place. 



Other things being equal, we should expect the violence of the 

 outburst to be directly proportional to the length of contact of the 

 igneous magma with water-bearing strata, or, in other words, the 

 longer the quiescence the greater the violence in the subsequent 

 eruption. 



The diffusion or solution of water through the igneous magma 



