312 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
When eruptions are of a less violent explosive character the first 
part may be pumice and later a more compact and micro and par- 
tially 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 buffish- 
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 beep 
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 H,O 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 H,O 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 recombine 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 
H,O is absorbed, and igneous outflow shows little violence, so that 
lavas are the chief products. 
If the voleanic 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 H,O its tension will steadily rise. Its loss of 
heat energy will be very little, as the H,O 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 
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. 
