ORIGIN OF LITHOLOGICAL FEATURES. Sa 
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of the surface is mostly a silicate of iron, containing more iron than 
the rock below, and consequently of high specific gravity. The 
hypothesis does not appear to be satisfactory. Yet what better solu- 
tion can be offered of this difficult problem? ‘Trusting to the teach- 
ings of Kilauea, we proceed with such suggestions as have presented 
themselves. 
We have stated that the lavas are made up of materials of different 
degrees of fusibility, as well as of unlike specific gravity : it is also a 
fact which will be admitted that the volcanic action is promoted by 
vaporizable substances within the fused mass—partly sulphur, from 
the material fused, and partly water, which is constantly finding 
access to the fires. These are the data for our conclusions. 
1. The vaporizable substances tend to become vapours, and the 
greater their abundance, the more they inflate the lavas, lessening the 
specific gravity ; this effect increases rapidly the nearer they approach 
to the summit of the column, where the pressure is constantly less, so 
that the lava finally is actually much expanded by them. This effect 
produces, as explained on page 204, a slow rising of the lavas about 
the central part of the conduit, towards the surface, and accompanying 
this action, a descending current along the sides, though of less dis- 
tinctness. The ebullition at the surface is only the escape of these 
confined vapours. The large lake of Kilauea has the very motions 
here represented. ‘Thus far, therefore, there is no hypothesis. 
2. The material in fusion has been described as consisting of augite 
and feldspar, minerals of unequal fusibility.* Wherever the tempe- 
rature of the liquid mass begins to be less than that necessary to 
retain the feldspar in fusion, there the feldspar will commence to 
solidify, or will slowly stiffen in the midst of the fluid material made 
up of the other ingredients. In this state, the vapours ascending in 
the conduit, will urge upward the feldspar much less freely than the 
more liquid part of the lava; for the latter will yield more readily to 
the inflating vapours, and thus become lighter, and rise to the sur- 
face; and this process, going on constantly through the whole pro- 
* Whether the mineral augite is decomposed at the heat which melts feldspar, and at 
that temperature its constituents are in fusion in some different combination, rather than 
as augite itself, we cannot say. Neither is it essential to the explanation if feldspar is 
the more infusible ingredient. 
t The temperature of fusion, and that of solidification, do not appear to be identical in 
all minerals, as the temperature of the fluid mass may be carried some degrees below 
that required to commence fusion, before incipient solidification will appear, 
