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 microscope, 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 bet 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 carried 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 phenomena are exhibited by them which lend powerful support to this conclusion. [Illustration: Fig. 7.--Minute Cavities, containing Liquids, in the Crystals of Rocks.] [Sidenote: LIQUID CAVITIES IN CRYSTALS.]