Now, the study of the products of volcanoes has led geologists to recognise the true relations between glassy and crystalline rocks. In the amorphous mixture of various silicates which compose a glass, chemical affinity causes the separation of certain portions of definite composition, and these form the microliths or elements of which different crystalline minerals are built up. Under the influence of the crystalline forces, there is a great shaking or agitation in the mass, and the microliths of similar kind come together and become united, like the fragments in Ezekiel's valley of dry bones. 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. [Sidenote: 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.