A slice of volcanic glass under the microscope, showing well-developed microliths. (After Cohen.)
Our first slice, however (Fig. 41), tells us something more of their history, for the fact that they are arranged in lines shows that they have grown while the lava was flowing and carrying them along in streams. You will notice that each one has its greatest length in the direction of the lines, just as pieces of stick are carried along lengthways in a river. In the second specimen (Fig. 42) the microliths are much larger and the stream has evidently not been flowing fast, for they lie in all directions.
This is what we find in the upper part of the stream, but if we look at a piece of underlying lava we find that it is much more coarse-grained, and the magnifying-glass shows many crystals in it, as well as a number of microliths. For this lava, covered by the crust above, has remained very hot for a long time, and the crystals have had time to build themselves up out of the microliths and crystallites.
Still there is much glassy groundwork even in these lavas. If we want to find really stony masses such as porphyry and granite made up entirely of crystals we must look inside the mountain where the molten rock is kept intensely hot for long periods, as for example in the fissure g, Fig. 39.
Such fissures sometimes open out on the surface like the one I saw, and sometimes only penetrate part of the way through the hill; but in either case when the lava in them cools down, it forms solid walls called dykes which help to bind the loose materials of the mountain together. We cannot, of course, examine these in an active volcano, but there are many extinct volcanoes which have been worn and washed by the weather for centuries, so that we can see the inside. The dykes laid bare in the cliffs of Somma are old fissures filled with molten rock which has cooled down, and they show us many stony lavas; and Mr. Judd tells us of one beautiful example of a ruined volcano which composes the whole island of Mull in the Hebrides, where such dykes can be traced right back to a centre. This centre must once have been a mass of melted matter far down in the earth, and as you trace the dykes back deeper and deeper into it, the rocks grow more and more stony, till at last they are composed entirely of large crystals closely crowded together without any glassy matter between them. You know this crystalline structure well, for we have plenty of blocks of granite scattered about on Dartmoor, showing that at some time long ago molten matter must have been at work in the depths under Devonshire.
We see then that we can trace the melted rock of volcanoes right back—from the surface of the lava stream which cools quickly at the top, hurrying the crystallites and microliths along with it—down through the volcano to the depths of the earth, where the perfect crystals form slowly and deliberately in the underground lakes of white-hot rock which are kept in a melted state at an intense heat.
Fig. 43.
A piece of Dartmoor Granite, drawn from a specimen.
But I promised you that we would have no guesswork here, and you will perhaps ask how I can be certain what was going on in the depths when these crystals were formed. A few years ago I could not have answered you, but now chemists, and especially two eminent French chemists, MM. Fouqué and Levy, have actually made lavas and shown us how it is done in Nature.