Before we part from this subject of the classification of lavas according to their chemical composition, it will be well to point out that there exists a small group of lavas which stand quite by themselves, and cannot be referred to either of the classes we have indicated. They contain a smaller proportion of silica, and a much larger proportion of magnesia and oxide of iron than the other lavas, and may be made to constitute a small sub-group, to which we may apply the term of 'ultra-basic lavas.' Although much less widely distributed than the other varieties, they are, in some respects, as we shall presently have to point out, of far greater interest to the geologist than all the other kinds of lavas. [Sidenote: MINUTE STRUCTURE OF LAVAS.] We will now proceed to consider the facts which are brought to light concerning the nature of lavas, when they are studied by the aid of the microscope. Although most lavas appear at first sight to be opaque substances, yet it is easy to prepare slices of them which are sufficiently thin to transmit light. In such thin transparent slices we are able to make out, by the aid of the microscope, certain very interesting details of structure, which afford new and important evidence bearing on the mode of origin of these rocks. Host lavas are capable of being melted by the heat of our furnaces; but the different kinds of lava vary greatly in the degree of their fusibility. The basic lavas, or those with the smallest proportion of silica, are usually much more easily fusible than those which contain a high percentage of silica, the add lavas. Now, it is a very noteworthy circumstance, that when a lava is artificially fused it assumes on cooling very different physical characters to those which were presented by the original rock. If we examine the freshly-broken surface of a piece of lava, we shall, in most cases, find that it contains a great number of those regular-shaped bodies which we call crystals; in some cases these crystals are so small as to be scarcely visible to the naked eye, in others they may be an inch or more in length. Most lavas are thus seen to be largely made up of crystals of different minerals. The minerals which are usually contained in lavas are quartz, the various kinds of felspar, augite, hornblende, the different kinds of mica, olivine, and magnetite. But when a piece of lava is melted in a furnace, all these crystalline minerals disappear, and the resulting product is the homogeneous substance which we call glass. If, as many suppose, lavas acquire the fluidity which they possess when issuing from volcanic vents as the result of simple fusion it is strange that artificially fused lavas do not agree more closely in character with the natural products.