Common flower pots and the tiles used in draining wet land are not glazed, as hard-burned bricks are. The baking of these clay things is done with much less heat. They are left somewhat porous. But the tiles of roofs are baked harder, and get a surface glaze by the melting of the glassy particles of the sand. As bricks vary in colour and quality according to the materials that compose them, so the metamorphic rocks differ. The white sand one sees on many beaches is largely quartz. This is the substance of pure, white sandstone. Metamorphism melts the silica into a glassy liquid cement; the particles are bound close together on cooling. The rock becomes a white, granular quartzite, that looks like loaf sugar. If banded, it is called gneiss. Such rocks take a fine polish. Pure limestone is also white and granular. When metamorphosed by heat, it becomes white marble. The glassy cement that holds the particles of lime carbonate shows as the glaze of the polished surface. It is silica. One sees the same mineral on the face of polished granite. Clays are rarely pure. Kaolin is a white clay which, when baked, becomes porcelain. China-ware is artificially metamorphosed kaolin. In the early rocks the clay beds were transformed by heat into jasper and slates. In beds where clay mingled with sand, in layers, gneiss was formed. If mica is a prominent element, the metamorphic rock is easily parted into overlapping, scaly layers. It is a mica schist. If hornblende is the most abundant mineral, the same scaly structure shows in a dark rock called hornblende schist, rich in iron. A schist containing much magnesia is called serpentine. The bricks of the wall, the tiles on the roof, the flower pots on the window sill, and the dishes on the breakfast table, are examples of metamorphic rocks made by man's skill, by the use of fire and water acting on sand and clay. Pottery has preserved the record of civilization, from the making of the first crude utensils by cave men to the finest expression of decorative art in glass and porcelain. The choicest material of the builder and the sculptor is limestone baked by the fires under the earth's crust into marble. The most enduring of all the rocks are the foundation granite, and the metamorphic rocks that lie next to them. Over these lie thick layers of sedimentary rocks laid down by water. In them the record of life on the earth is written in fossils. THE AIR IN MOTION