Volcanoes: What They are and What They Teach

by John W. Judd

In the Frontispiece, No. 1 illustrates the appearance presented by these volcanic glasses when examined with a high power of a microscope. Through a glassy base is seen a number of diffused nebulous patches, which are in places resolved into definite particles.

These minute particles of definite form, which the microscope has revealed in the midst of the glassy portions of lava, have received the name of microliths, or crystallites. The study of the characters and mode of arrangement of these microliths or crystallites has in recent years thrown much new light on the interesting problems presented by lavas.

In some glassy lavas the microliths or crystallites, instead of being indiscriminately diffused through the mass of the base or groundmass, are found to be collected together into groups of very definite form. In No. 2 of the Frontispiece we have a section of a glassy rock in which the crystallites have united together, so as to build up groups presenting the most striking resemblance to fronds of ferns. Around these groups spaces of dear glass have been left by the gathering up of the crystallites, which in other parts of the mass are seen to be equally diffused through it. In this formation of groups of microliths we cannot but recognise the action of those crystalline forces, which on frosty mornings cover our windows with a mimic vegetation composed of icy particles.

In other cases, again, the crystallites scattered through the glassy portions of lavas unite in radial groups about certain centres, and thus build up globular masses to which the name of 'sphærulites' has been given. No. 3 in the Frontispiece illustrates the formation of these sphærulites.

Now, a careful study of the microliths or crystallites has proved that they are the minute elements of which those wonderfully beautiful objects which we call crystals are built up. In some cases we can see that the crystallites are becoming united together in positions determined by mathematical laws, and the group is gradually assuming the outward form and internal structure of a crystal. In other cases crystals may be found which are undergoing a disintegrating action, and are then seen to be made up of minute elements similar to the crystallites or microliths of glassy rocks.

CRYSTALLITES AND CRYSTALS.

The conclusion is confirmed by the fact that if we take an artificially fused lava and allow it to cool slowly, it will be found that the glassy mass into which it has resolved itself contains numerous crystallites. If the cooling process be still further prolonged, these crystallites will be found to have united themselves into definite groups, and sometimes distinct crystals are formed in the mass; under these circumstances the rock frequently loses its glassy appearance and assumes a stony character.

In connection with this subject, it may be mentioned that some years ago a very ingenious invention was submitted to trial in the Works of the Messrs. Chance, of Birmingham. It had been suggested that if certain lavas of easy fusibility were melted and poured into moulds, we might thus obtain elaborately ornamented stone-work, composed of the hardest material, without the labour of the mason. The molten rock when quickly cooled was found to assume the form of a black glass, but when very slowly cooled passed into a stony material. Unfortunately, it was found that this material did not withstand the weather like ordinary building stones, and, in consequence, the manufacture had to be abandoned.

Now, the study of the products of volcanoes has led geologists to recognise the true relations between glassy and crystalline rocks.

In the amorphous mixture of various silicates which compose a glass, chemical affinity causes the separation of certain portions of definite composition, and these form the microliths or elements of which different crystalline minerals are built up. Under the influence of the crystalline forces, there is a great shaking or agitation in the mass, and the microliths of similar kind come together and become united, like the fragments in Ezekiel's valley of dry bones.