4. Ordinary scoria, common about cinder cones of stony tex ture. The vesicles constitute sixty to ninety-five per cent. of the mass.
5. Spongy thread lace scoria, existing as a layer a foot thick over the southwest part of Kilauea. The vesicles are very coarse and constitute ninety-eight to ninety-nine per cent. of the mass. They are polygonal with twelve and fourteen sides and frequently distorted by pressure. Some of the holes are half a cubic inch in bulk.
Very little water is required for most of this vesiculation ; no more than that of molecular diffusion. When a specimen con taining forty per cent. of vesicles has its specific gravity deter mined and compared with that of the same material solid, it is found to be as 1.88 to 2.98. The required water is hence .0003 per cent. of the bulk or .000i of the weight of the mass. The amount of moisture required to produce the vesiculation of the thread lace scoria was determined to be 3.125 per cent. of the bulk or 1.1 per cent. by weight.
Some have conceived that the downward ingress of water would be checked violently by the intense heat at great depths. The
best authors do not accept that view ; partly because when the tem perature of the critical point of water (773° F.) is reached, dis sociation takes place, and there may be an attraction rather than a repulsion. The absorption of the water will increase the bulk of the lava, so that there will be a greater pressure in the lower part of the conduit, perhaps enough to force the material to the surface : and thus vesiculation may be an important element in producing projectile results.
It has been noticed repeatedly that the liquid lava enlarges its area by dissolving its retaining walls. Floating islands and debris cones have also disappeared ; and it is a question whether the greater part of the calderas have not been enlarged in this way. Heated silicates possess greater powers of dissolving refractory substances, especially when under pressure; but this opens too large a subject to be discussed here.