Dr. Johnston- Lavis — Mechanism of Volcanic Action. 441 



fluidity, which -would explain in part why hydrated rocks are more 

 frequent amongst them than are acid ones, as more frequently they 

 would be cooled to consolidation before they found issue at the surface. 



In many cases, however, the tension of the magma will steadily 

 rise as it acquires more and more volatile matter from the surrounding 

 rocks. ^ A moment will be reached when the tension has gradually 

 attained such intensity that the earth's crust is rent by an extension 

 upwards of the fissure. This fissure may reach the surface and make 

 a new volcano, or the obstructing plug of an old one may be cleared 

 away. In either case an explosive eruption will result. 



Now the first portion to issue will be that part of the magma at the 

 top of the fissure that has been in contact for the longest time with 

 the more aquiferous rocks, and consequently will be richer in acquired 

 volatile materials than that below. It may be a pure glass, or 

 a certain number of crystals may have individualized under the 

 intratelluric conditions of slow cooling. Once free from compression 

 HgO, etc., will separate from the non-volatile silicates and oxides as 

 bubbles, undergo enormous expansion, escape in great part, and afford 

 the explosive agent in the ejection of the remaining fluid-froth still 

 holding much gas in the vesicles. This sudden expansion means 

 a tremendous loss of heat energy, and the vitreous matter is so rapidly 

 cooled that it has no time to individualize into microliths or crystals, 

 or the crystals already existing to grow in size. 



We know from the effect of pumice on combustible substances, as 

 wood, bread, cloth, etc., as at Pompeii, that nearly all the latent heat 

 has been used up in this expansion, so that only partial roasting has 

 resulted. Thus has been produced the first white light pumice of an 

 explosive eruption by the extremely rapid expansion and cooling of 

 a pure or nearly pure glass. 



As the upper contents of the volcanic canal blows out, that part of 

 the magma lower down follows. This next portion has been a shorter 

 time probably in contact with aquiferous rocks ; these latter, being 

 deeper, are usually poorer in Hg 0. The consequence is that this 

 second batch of magma will contain less volatile matter, expansion 

 will be slower, there will be less loss of heat required for expansion, 

 so that there will be time and other more favourable conditions for 

 part of the glass to individualize into microliths. (Compare 1, 2, 5, 6, 

 with 3, 4, 7, 8, Plates XXIV and XXV.) This second batch of magma 

 furthermore will have lost less heat energy in consequence of having 

 had less H, to dissolve, but also being deeply seated and in hotter 

 rocks it will have lost less heat by conduction. 



As we go deeper in the volcanic conduit these same differences will 

 be exaggerated, so that as the magma escapes almost all the glass 

 may be converted into microliths so as to leave little material to hold 

 them together, whilst the evolution of volatile gases will be sufiicient 



• I use H2 not to assume any special physical state of that substance. I also 

 refer to it specially as being the principal volatile material of igneous magma ; but 

 I fully appreciate the salts and gases clerived from their decomposilion, as likewise 

 the rarer materials that were acquired by and can be separated from igneous rocks 

 by a high temperature, to which Monsieur Brun and others have furnished us with 

 such interesting details by their studies. 



