Although we cannot see this process taking place under our eyes, in a mass of lava, yet we may study specimens in which the action has been arrested in its different stages. In order to understand the development of an acorn into an oak-tree, it is not necessary to watch the whole series of changes in a particular case. A visit to an oak-thicket, in which illustrations of every stage of the transformation may be found, will afford us equally certain information on the subject.
In the same way by the examination of such a series of rock-sections as that represented in the Frontispiece, we may understand how, in the midst of a mass of mixed silicates constituting a natural glass, the separation of microliths takes place; these unite into groups which are the skeletons of crystals, and finally, by the filling up of the empty spaces in these skeletons, complete crystals are built up. The series of operations may, however, be interrupted at any stage, and this stage we may have the chance of studying.
GLASSY AND CRYSTALLINE LAVAS.
We are able, as we shall show in a future chapter, to examine many rock-masses that have evidently formed the reservoirs from which volcanoes have been supplied, and others that fill up the ducts which constituted the means of communication between these subterranean reservoirs, and the surface of the earth. Now in these subterranean regions the lavas have been placed under conditions especially favourable for the action of the crystalline forces—they must have cooled with extreme slowness, and they must have been under an enormous pressure, produced in part by the weight of the superincumbent rocks, and in part by the expansive force of the imprisoned steam. We are not, therefore, surprised to find that in these subterranean regions, the lavas, while retaining the same chemical composition, have assumed a much more perfectly crystalline condition. In some cases, indeed, the whole rock has become a mass of crystals without any base or groundmass at all.
An examination of the Frontispiece will illustrate this perfect gradation from the glassy to the crystalline condition of lavas. No. 1 represents a glass through which microliths or crystallites of different dimensions and character are diffused. In Nos. 2 and 3, these crystallites have united to form regular groups. In No. 4, which may be taken as typical of the features presented by most lavas, we have a glassy groundmass containing microliths (a 'crypto-crystalline base'), through which distinct crystals are distributed. Nos. 5 and 6 illustrate the characters presented by lavas which have consolidated at considerable depths beneath the surface; in the former we have a mans of small crystals (a 'micro-crystalline base') with larger crystals scattered through it; while the latter is entirely made up of large crystals without any trace of a base or groundmass.
Now, as all lavas are found sometimes assuming the glassy condition at the surface, so when seen in the masses which have consolidated with extreme slowness, and under great pressure, in subterranean regions, the same materials are found in the condition of a rock which is built up entirely of crystals. Chemists have found that artificial mixtures of silicates in which soda and potash are present in considerable quantities, have a great tendency to assume the glassy condition on cooling from a state of fusion, and glass manufacturers are always careful to use considerable proportions of the alkalis as ingredients, in making glass. It is found, in like manner, that those lavas which contain the largest portion of the silicates of soda and potash (the 'acid lavas') most frequently assume the condition of a natural glass.
Geologists have given distinct names to the glassy and the perfectly crystalline conditions of the different kinds of lavas, the glassy varieties being found in masses which have cooled rapidly near the surface, and the crystalline varieties in masses which have cooled slowly at great depths. The names of these two conditions of the five great classes into which we have divided lavas are as follows:—
HIGHLY CRYSTALLINE IGNEOUS ROCKS.
| Crystalline Forms. | Lavas. | Glassy Forms. | |
| Granite | Rhyolite |  | Obsidian. |
| Syenite | Trachyte | ||
| Diorite | Andesite | ||
| Miascite | Phonolite | ||
| Gabbro | Basalt | Tachylyte. | |
As vitreous rocks have little in their general appearance to distinguish them from one another, the glassy forms of the first four classes of lava have not hitherto received distinct names, but have been confounded together under the name of obsidian. If we determine the specific gravities of rocks having the same composition but different structures, we shall find that they become heavier in proportion as the crystalline structure is developed in them. Thus gabbro is heavier, but tachylyte is lighter than basalt, bulk for bulk, though all have the same chemical composition.