"Some physicists have asserted that a globe of liquid matter radiating its heat into space, would tend to solidify both at the surface and the centre at the same time. The consequence of this action would be the production of a sphere with a solid external shell and a solid central nucleus, but with an interposed layer in a fluid or semi-fluid condition. It has been pointed out that if we suppose the solidification to have gone so far as to have caused the partial union of the interior nucleus and the external shell, we may conceive a condition of things in which the stability and rigidity is sufficient to satisfy both geologists and astronomers, but that in still unsolidified pockets or reservoirs, filled with liquefied rock, between the nucleus and the shell, we should have a competent cause for the production of the volcanic phenomena of the globe. In this hypothesis, however, it is assumed that the cooling at the centre and the surface of the globe would go on at such rate that the reservoirs of liquid material would be left at a moderate depth from the surface, so that easy communication could be opened between them and volcanic vents."
I must caution you, however, not to think that the above theory of volcanoes is accepted by all scientific men. On the contrary, there are many who believe that the earth is solid throughout because it has completely lost its original heat; that it is only comparatively small areas that are to be found filled with molten or at least highly heated material. But these opinions are held largely by those who have given their attention almost entirely to the phenomena of earthquakes, or who base their reasonings on mathematical grounds only and have not sufficiently considered the phenomena of volcanoes. Since, however, they can be better understood after we have explained the phenomena of earthquakes, we will defer their discussion to the last chapters of this book.
[CHAPTER XX]
SOME FORMS OF LAVA
In describing the wonders of volcanoes, we must not fail to say something of the many remarkable forms that lava is capable of assuming.
All volcanic lavas contain large quantities of an acid substance known as silica, or what is known better as quartz sand. This material exists in lava combined chemically with various substances called bases, the principal of which are alumina, magnesia, lime, iron, potash, and soda.
Although there are many kinds of lava, yet all lavas can be arranged under three great classes according to the quantity of silica they contain.
Acid lavas are those in which the quantity of silica is greatest. In these lavas the silica, which varies from 66 to 80%, is combined with small quantities of lime or magnesia, and comparatively large quantities of potash or soda. Some of the most important varieties of acid lavas are known as trachytes, andesites, rhyolites, and obsidians.