136 Scientific Proceedings, Royal Dublin Society. 



verify the same facts at Roccamonfina, Mt. Vultura, Monte Nuovo, 

 San Stefano, Ventotene, Ischia, and many other volcanoes, in at 

 least a hundred different eruptions. In the second part of this 

 paper I propose to bring forward typical examples from each 

 locality in illustration of this. For the present, however, we may 

 state the divisions as follow : — 



1. Ejection of vitreous froth, which rapidly solidifies, as pumice ; 

 all the minerals that occur crystallized are of plutonic separation, 

 as sanidine, biotite, amphibole, &c. 



2. Microcrystalline pumice, in which surface cooling has 

 produced pyroxene and leucite. The amount of vitreous base 

 diminishes as we reach the top of this division, and is replaced 

 by formed material. 



3. The pumiceous ash-bed in which the cementing vitreous 

 base is nearly all destroyed, so that cohesion has become so feeble 

 that the formed matter separates, producing an ash composed of 

 crystals and microliths. The difference is very similar to the 

 results of crystallization of a salt in the form of large crystals by 

 a slow process, or, in the preparation of the granular state, by a 

 quick one, as table salt, pure Ferrous sulphate, and oxalic acid, as 

 they are met with in commerce. 



The increase of the percentage of silica has the effect of 

 rendering acid-rocks less easily crystallizable, just as the amor- 

 phous form of sugar retards crystallization of other bodies with 

 which it is mixed. For the same cause, the viscosity of the rock 

 is increased, so that the escape of the enclosed gaseous bubbles 

 takes place with greater difficulty; and, as a result, the pumiceous 

 character is far more common amongst such rocks. 



Mode of Formation and Structure of Scoria. — This product, 

 which is often erroneously grouped together with pumice, is that 

 spongy variety of lava which covers or underlies a stream. When 

 the magma does not contain sufficient volatile constituents to tear 

 it asunder before it issues from the volcanic vent, it will pour 

 down the slopes of the cone, giving up what remnants of aqueous 

 matter are still dissolved within it. Should this be considerable in 

 amount, and the temperature of the lava rather low, in basic 

 examples we shall get an irregular broken-up cinder-like mass, 

 that will continue to float on the surface, and cover it in some 



