Through Magic Glasses and Other Lectures / A Sequel to The Fairyland of Science

by Arabella B. Buckley

By using powerful furnaces and bellows they have succeeded in getting temperatures of all degrees, from a dazzling white heat down to a dull red, and to keep any temperature they like for a long time, so as to imitate the state of a mass of melted rock at different depths in the earth, and in this way they have actually made lavas in their crucibles. For example, there is a certain whitish rock common in Vesuvius called leucotephrite,[4] which is made up chiefly of crystals of the minerals called leucite, Labrador felspar, and augite. This they proposed to make artificially, so they took proper quantities of silica, alumina, oxide of iron, lime, potash, and soda, and putting them in a crucible, melted them by keeping them at a white heat. Then they lowered the temperature to an orange-heat, that is a heat sufficient to melt steel. They kept this heat for forty-eight hours, after which they took out some of the mixture and, letting it cool, examined a slice under the microscope. Within it they found crystals of leucite already formed, showing that these are the first to grow while the melted rock is still intensely hot. The rest of the mixture they kept red-hot, or at the melting-point of copper, for another forty-eight hours, and when they took it out and examined it they found that the whole of it had been transformed into microliths of the two other forms of crystals, Labrador felspar and augite, except some small eight-sided crystals of magnetite and picotite which are also found in the natural rock.

There is no need for you to remember all these names. What I do want you to remember is, that, at the different temperatures, the right crystals and beginnings of crystals grew up to form the rock which is found in Vesuvius. And what is still more interesting, they grew exactly to the same stages as in the natural rock, which is composed of crystals of leucite and microliths of the two other minerals.

This is only one among numerous experiments by which we have learnt how volcanic rocks are formed and at what heat the crystals of different substances grow. We are only as yet at the beginning of this new study, and there is plenty for you boys to do by and by when you grow up. Many experiments have failed as yet to imitate certain rocks, and it is remarkable that these are usually rocks of very ancient eruptions, when perhaps our globe may have been in a different state to what it is now; but this remains for us to find out.

Meanwhile I have still another very interesting slide to show you which tells us something of what is going on below the volcano. Under the third microscope I have put a slice of volcanic glass (Fig. 44) in which you will see really large crystals with dark bands curving round them. These crystals have clearly not been formed in the glass while the lava was flowing, first because they are too large to have grown up so rapidly, and secondly because they are broken at the edges in places and sometimes partly melted. They have evidently come up with the lava as it flowed out of the mountain, and the dark bands curving round them are composed of microliths which have been formed in the flow and have swept round them, as floating straws gather round a block of wood in a stream.

Such crystals as these are often found in lava streams, and in fact they make a great difference in the rate at which a stream flows, for a thoroughly melted lava shoots along at a great pace and often travels several miles in a very short time; but an imperfectly melted lava full of crystals creeps slowly along, and often does not travel far from the crater out of which it flows.

Fig. 44.

Slice of volcanic glass under the microscope, showing large included crystals brought up from inside the volcano in the fluid lava. The dark bands are lines of microliths formed as the lava cooled. (J. Geikie.)

So you see we have proof in this slice of volcanic glass of two separate periods of crystallisation—the period when the large crystals grew in the liquid mass under the mountain, and the period when the microliths were formed after it was poured out above ground. And as we know that different substances form their crystals at very different temperatures, it is not surprising that some should be able to take up the material they require and grow in the underground lakes of melted matter, even though the rest of the lava was sufficiently fluid to be forced up out of the mountain.

And here we must leave our lava stream. The microscope can tell us yet more, of marvellous tiny cavities inside the crystals, millions in a single inch, and of other crystals inside these, all of which have their history; but this would lead us too far. We must be content for the present with having roughly traced a flow of lava from the depths below, where large crystals form in subterranean darkness, to the open air above, where we catch the tiny beginnings of crystals hardened into glassy lava before they have time to grow further.