A very similar structure is often seen in certain glassy lavas, when they are examined in thin sections under the microscope. Such glassy lavas exhibit the peculiar lustre of mother-of-pearly doubtless in consequence of the interference of light along the cracks. Lavas exhibiting this character are known to geologists as 'perlites.' The perlitic structure has been produced artificially by Mr. Grenville Cole in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically deposited silica. See [fig. 31].
A thick lava-stream must take an enormous period to cool down—probably many hundreds or even thousands of years. It is possible to walk over lava-streams in which at a few inches below the surface the rock is still red-hot, so that a piece of stick is lighted if thrust into a crack. Lava is a very bad conductor of heat, and loose scoriæ and dust are still worse conductors. During the eruption of Vesuvius in 1872, masses of snow which were covered with a thick layer of scoriæ, and afterwards by a stream of lava, were found three years afterwards consolidated into ice, but not melted. The city of Catania is constantly supplied with ice from masses of snow which have been buried under the ejections of Etna.
During the cooling down of lavas, the escape of steam and various gases gives rise to the deposition of many beautiful crystalline substances in the cavities and on the surfaces of the lava. Deposits of sulphur, specular-iron, tridymite, and many other substances are often thus produced, and the colour and appearance of the rock-masses are sometimes completely disguised by these surface incrustations, or by the decomposition of the materials of the lava by the action of the add gases, and vapours upon it.
SINKING OF SURFACES OF LAVA-STREAMS.
Very frequently the surface of a lava-stream becomes solid, while the deeper portions retain their fluid condition; under such circumstances the central portions may flow away, leaving a great hollow chamber or cavern. In consequence of this action, we not unfrequently find the upper surface of a lava-current exhibiting a depression, due to the falling in of the solidified upper portions when the liquid lava has flowed away and left it unsupported, as in [fig. 32].
Fig. 32.—Transverse section of a Lava stream. (The dotted line indicates the original surface.)
CHAPTER V
THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS.
Near the high-road which passes between the towns of Eger and Franzenbad in Bohemia, there rises a small hill known as the Kammerbühl (see [fig. 33]), which has attracted to itself an amount of interest and attention quite out of proportion to its magnitude or importance. During the latter part of the last century and the earlier years of the present one, the fiercest controversies were waged between the partisans of rival schools of cosmogony over this insignificant hill; some maintaining that it originated in the combustion of a bed of coal, others that its materials were entirely formed by some kind of 'aqueous precipitation,' and others again that the hill was the relic of a small volcanic cone.
Among those who took a very active part in this controversy was the poet Goethe, who stoutly maintained the volcanic origin of the Kammerbühl, styling it 'a pocket edition of a volcano.' To Goethe belongs the merit of having suggested a Very simple method by which the controversies concerning this hill might be set at rest: he proposed that a series of excavations should be undertaken around the hill, and a tunnel driven right under its centre.