The term _lapilli_, or sometimes _rapilli_, is applied to the ejected lava fragments when of the average size of a finger joint. This is the material which still partially covers the unexhumed portions of the city of Pompeii. Volcanic _sand_, _ash_, and _dust_ are terms applied in order to increasingly fine particles of the ejected lava. The finest material, the volcanic dust, is often carried for hundreds and sometimes even for thousands of miles from the crater in the high-level currents of the atmosphere. Inasmuch as this material is deposited far from the crater and in layers more or less horizontal, such material plays a small rôle in the formation of the cinder cone. The coarser sands and ash, on the other hand, are the materials from which the cinder cone is largely constructed. The manner of formation and the structure of cinder cones may be illustrated by use of a simple laboratory apparatus (Fig. 119). Through an opening in a board, first white and then colored sand is sent up in a light current of air or gas supplied from suitable apparatus. The alternating layers of the sand form in the attitudes shown; that is to say, dipping inward or toward the chimney of the volcano at all points within the crater rim, and outward or away from it at all points outside (Fig. 119). If the experiment is carried so far that at its termination sand slides down the crater walls into the chimney below, the inward dipping layers will be truncated, or even removed entirely, as shown in Fig. 119 _b_. [Illustration: FIG. 120.—Diagram to show the contrast between a lava dome and a cinder cone. _AAA_, cinder cone; _BabC_, lava dome; _DE_, line of low cinder cones above a fissure (after Thoroddsen).] =The profile lines of cinder cones.=—The shapes of cinder cones are notably different from those of lava mountains. While the latter are domes, the mountains constructed of cinder are conical and have curves of profile that are concave upward instead of convex (Fig. 120). In the earlier stages of its growth the cinder cone has a crater which in proportion to the height of the mountain is relatively broad (Fig. 99, p. 104). [Illustration: FIG. 121.—Mayon volcano on the island of Luzon, P.I. A remarkably perfect high cinder cone.]