PROCEEDINGS OP THE GEOLOGICAL SOCIETY. IO5 



The extrusion or eruption of igneous matter into the atmosphere 

 was then studied, and it was shown that one of the irregularities in 

 eruptive activity is due to the varying conditions which different parts 

 of the magma have undergone before reaching the surface. The 

 main varieties of volcanic outbursts were discussed : the violence is 

 dependent upon the amount of magma and its contained water, 

 which on relief of pressure expands with enormous rapidity, tending 

 to reduce the whole mass to 100°. This process in explosive erup- 

 tions accounts for the pumice and the vitreous structure of this type 

 of eruption. The remarkable fact that the pumice-beds resulting 

 from any basic explosive eruption are vitreous towards the bottom 

 and become more crystalline as we approach the surface, was shown 

 to be due to the slower cooling in consequence of less absorption of 

 heat in converting water into steam as the eruption progresses. 



The various conditions which bring about the extinction of a vol- 

 cano were then pointed out. The higher the volcano the more violent 

 will be its eruptions ; but the intervals wiU be greater. The mecha- 

 nism of a lateral outburst was then demonstrated. The amount of 

 lava escaping from a given point laterally is far more than that 

 contained in the chimney above, which is due to the welling-up of 

 the portion below when relieved from the pressure of the column 

 that occupied the upper part of the chimney. 



The discussion of the effect of the presence of volatile matter 

 in modifying the composition and structure of igneous rocks is so 

 long and intricate that it is impossible to render it in abstract ; the 

 eruptive phases of Monte Somma, Eoccamonfina, Monte Yulture, 

 Yentotene, and Monte Nuovo are given as examples. The difference 

 of the rocks produced will depend on — 



(a) Composition of the original magma. 



(6) Pre-eruptive temperature of the same. 



(c) Amount of enclosed volatile matter. 



{d) Amount of pre-eruptive crystallization. 



{e) Rapidity of ejection. 



(/) Height of projection. 



{g) Temperature of the atmosphere. 



In the first appearance of a volcano, or the reawakening of one, 

 vitreous pumiceous fragmentary products first appear, and pass by 

 way of more microcrystalline pumice, pumiceous scoria, to actual 

 lava outflows. Scoria differs from pumice in that the vesicular 

 structure is derived from other portions of the magma, whereas in 

 pumice it is formed, where found, by the intermolecular expansion 

 and union of the resulting steam into bubbles. Yesicularity of 

 lavas depends upon the amount of dissolved water and the viscosity 

 of the mass. 



Yolcanic ashes are the result either of the complete reduction of 

 the magma to a powder by the enormous and rapid expansion of the 

 magma in explosive eruptions, or of the grinding-up of the accessory 

 ejectamenta derived from the crater-sides or from accidental ejecta- 

 menta that may lie beneath the volcano, and into which the apex 



