GLASSY AND CRYSTALLINE LAVAS. 57 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 consoli dated at considerable depths beneath the surface; in the former we have a mass of small crystals (a 'microcrystalline 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:: 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. Nor are the crystals contained in lavas less worthy of careful study, by the aid of the miscroscope, than the more or less glassy groundmass in which they are embedded. Mr. Sorby has shown that the crystals found in lavas, exhibit many interesting points of difference from those which separate out in the midst of a mass of the same rock, when it has been artificially melted and slowly cooled. There are other facts which also point to the conclusion that, while the glassy groundmass of lavas may have been formed by cooling from a state of fusion, the larger and well-formed crystals in these lavas must have been formed under other and very different conditions. The larger crystals in lavas exhibit evidence of having been slowly built up in the midst of a glassy mass, containing crystallites and small crystals. We can frequently detect evidence of the interruptions which have occurred in the growth of these crystals in the concentric zones of different colour or texture which they exhibit; and portions of the glassy base or groundmass are often found to have been caught up and enclosed in these crystals during their growth. But when we find, as in the porphyritic pitchstones, a glassy base containing only minute crystallites, through which large and perfectly formed crystals are distributed, we can scarcely doubt that the minute crystallites and the larger crystals have separated from the base under very different conditions. This is indicated by the fact that we detect in these cases no connecting links between the embryo microliths and the perfect crystals; and a confirmation of the conclusion is seen in the circumstance that many of the crystals are found to have suffered injury as if from transport, their edges and angles being rounded and abraded, and portions being occasionally broken off from them. Hence we are led to conclude that the larger crystals in lavas were probably separated from the amorphous mass in the subterranean reservoirs beneath the volcano, and were floated up to the surface in the midst of the liquefied glassy material which forms the groundmass of lavas. When we come to examine these crystals more closely, we find that certain very curious phe |