Page:Popular Science Monthly Volume 20.djvu/364

350 the characters presented by lavas which have consolidated at considerable depths below the surface.

The microliths have been proved to be the minute elements from which common crystals are built up; and cases occur in which a group of them may be seen gradually assuming the outward form and internal structure of a crystal, and 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 microliths.

The same materials which go to form a lava may assume a glassy condition, or that of a rock built up entirely of crystals. Geologists have given distinct names to the glassy and crystalline forms of lavas, which correspond with the five great classes into which lavas have been divided as follows:

The obsidians do not exhibit enough differences to demand a distinction.

When the large crystals imbedded in granitic rocks and in some lavas are examined with the microscope, they are often found to contain numerous minute cavities, each of which resembles a small spirit level, having a quantity of liquid and a bubble of gas within it.

In No. 1 of Figure 2 a group of such cavities is represented, one of which is full of liquid, while two others are quite empty; the remaining cavities all contain a liquid with a moving bubble of gas. In No. 2 two larger cavities are shown, containing a liquid and a bubble of gas. In Nos. 3, 4, and 5 the liquid in the cavities contains, besides the bubbles, several minute crystals; and in No. 6 we have a cavity containing two liquids and a bubble.

In the largest of such cavities the bubble may be observed to change its place when the position of the cavity is altered, so as always to lie at the upper side, just as in a spirit-level; while in the smallest cavities the bubbles appear to be endowed with a power of spontaneous movement, and are seen continually oscillating from side to side and from end to end of the hollow, as in Figure 3, where the dark line shows the path pursued by the bubble. These cavities are exceedingly minute, and so numerous that there must be millions of them in some crystals; indeed, in certain cases, as we increase the magnifying power of the microscope, new and smaller ones continually become visible; and it has been estimated that in some instances the number of them amounts to from one thousand million to ten thousand million in a cubic inch of space. In many cases the included liquid is water containing saline matters in solution,