1, Slaggy crust, formed chiefly of scoriæ of a glassy nature. 2, Middle portion where crystals form. 3, Slaggy crust which has slipped down and been covered by the flow.
You would be surprised to see how quickly the top hardens, so that you can actually walk across a stream of lava a day or two after it comes out from the mountain. But you must not stand still or your shoes would soon be burnt, and if you break the crust with a stick you will at once see the red-hot lava below; while after a few days the cavities become filled with crystals of common salt, sulphur or soda, as the vapour and gases escape.
Then as time goes on the harder minerals gradually crystallise out of the melted mass, and iron-pyrites, copper-sulphate, and numerous other forms of crystal appear in the lower part of the stream. In the clinkers above, where the cooling goes on very rapidly, the lavas formed are semi-transparent and look much like common bottle-glass. In fact, if you take this piece of obsidian or volcanic glass in your hand, you might think that it had come out of an ordinary glass manufactory and had nothing remarkable in it.
Fig. 41.
A slice of volcanic glass showing the lines of crystallites and microliths which are the beginnings of crystals. (J. Geikie.)[2]
But the microscope tells another tale. I have put a thin slice under the first microscope, and this diagram (Fig. 41) shows what you will see. Nothing, you say, but a few black specks and some tiny dark rods. True, but these specks and rods are the first beginnings of crystals forming out of the ground-mass of glassy lava as it cools down. They are not real crystals, but the first step toward them, and by a careful examination of glassy lavas which have cooled at different rates, they have been seen under the microscope in all stages of growth, gradually building up different crystalline forms. When we remember how rapidly the top of many glassy lavas cool down we can understand that they have often only time to grow very small.
The smaller specks are called crystallites, the rods are called microliths.[3] Under the next microscope you can see the microliths much more distinctly (Fig. 42) and observe that they grow in very regular shapes.
Fig. 42.
