A careful examination of different kinds of lavas, however, will show that they vary very greatly in character among themselves. Some lavas are as perfectly glassy in structure as those which have been artificially fused, while others contain great numbers of crystals, which may sometimes be of very large size. If we prepare thin transparent slices of these different kinds of lavas, and examine them by the aid of the microscope, we shall find that lavas are made up of two kinds of materials, a base or groundmass of a glassy character, and distinct crystals of different minerals, which are irregularly distributed through this glassy base, like the raisins, currants, and pieces of candied peel in a cake. In some cases the glassy base makes up the whole mass of the rock; in others, smaller or larger numbers of crystals are seen to be scattered through a glassy base; while in others again the crystals are so numerous that the presence of an intervening glassy base or groundmass can only be detected by the aid of the microscope. [Sidenote: STUDY OF LAVAS WITH THE MICROSCOPE.] If thin slices of the glassy materials of lavas be examined with high magnifying powers, new and interesting facts are revealed. Through the midst of the clear glassy substance cloudy patches are seen to be diffused; and, if we examine them with a still higher power, these cloudy patches resolve themselves into innumerable particles, some transparent and others opaque, having very definite outlines. At the same time fresh cloudy patches are brought into view, which can only be resolved by yet higher powers of the microscope. In examining these natural glasses by the aid of the microscope, we are forcibly reminded of what occurs when the 'Milky Way' and some other parts of the heavens are studied with a telescope. As the power of the instrument is increased the nebulous patches are resolved into distinct stars, but fresh nebulous masses come into view, which are in turn resolved into stars, when higher powers of the instrument are employed. In the Frontispiece, No. 1 illustrates the appearance presented by these volcanic glasses when examined with a high power of a microscope. Through a glassy base is seen a number of diffused nebulous patches, which are in places resolved into definite particles. These minute particles of definite form, which the microscope has revealed in the midst of the glassy portions of lava, have received the name of microliths, or crystallites. The study of the characters and mode of arrangement of these microliths or crystallites has in recent years thrown much new light on the interesting problems presented by lavas.