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



out during an explosive eruption. This third or uppermost portion 

 consists of a coarse or fine dust (ash), and when examined micro- 

 scopically is seen to be to a lai'ge extent composed of detached 

 microliths and loose crystals mixed with a large quantity of pulverized 

 accessory and accidental ejecta. Whenever one examines the ejecta of 

 explosive eruptions the same order is found. The above-mentioned 

 characters are better seen in the case of very fluid intermediate or 

 basic magmas, and I have given figures of the ejecta of an earlier 

 explosive eruption of Somma- Vesuvius, Phase III, which is free from 

 leucite (PI. XXIV, Figs. 1-4). 



When eruptions are of a less violent explosive character the first 

 part may be pumice and later a more compact and micro- and partially 

 crystalline rock may issue, such, for instance, as the black pumiceous 

 trachytic scoria ejected by the last efforts of Monte Nuovo, the main 

 mass of the cone having been built up of a light huffish -white 

 trachytic pumice. 



In still less marked explosive eruptions, or where a large amount 

 of material is ejected extending over some time, the final product may 

 issue as a continuous mass and constitute a lava. 



What, then, is the interpretation of this regular succession of ejecta 

 having different characters ? We know that the surface rocks of the 

 earth's crust are as a body usually very aquiferous, and that as one 

 descends the rocks become drier and drier. All the water has been 

 squeezed out by superincumbent pressure. Of course, we know that 

 according to the nature and composition of the rocks the depth to 

 which aquiferous material extends will be extremely variable. 



Let us figure to ourselves what would take place in a mass of 

 fused silicates and oxides filling a fissure extending up through non- 

 aquiferous into more and more aquiferous rocks. The prolonged 

 contact would result in the gradual solution of the Hg of the 

 aquiferous strata in the fused paste, just as carbonic acid would be 

 dissolved under pressure, but at ordinary temperature, in water. In 

 the former case the critical point of Ho does not come into the 

 question. We know little of the temperature and pressure that 

 this compound can exist at when dissolved in silicates and oxides. 

 Furthermore, even if dissociated probably its components could pass 

 into solution and re-combine again when temperature was lowered 

 sufficiently. A careful study of volcanic action leads me to believe 

 this process to be a slow one, so that if a fairly regular flow of melted 

 rock takes place up the fissure through the aquiferous strata little 

 Hg is absorbed, and igneous outflow shows little violence, so that 

 lavas are the chief products. 



If the volcanic canal has never reached the surface, or is cut off 

 from it by an old plug of solidified ejecta, then as the igneous magma 

 acquires more and more Hg its tension will steadily rise. Its loss of 

 heat energy will be very little, as the Hg and other volatile matters 

 it has dissolved occupy a small volume. Still, in certain cases the 

 magma may, as the result of different sources of heat loss, undergo 

 complete cooling and consolidation. Not unlikely many hydrated 

 rocks owe their origin to this cause. All evidence points to the heat 

 energy or specific heat of basic rocks being lower in relation to their 



