For most practical purposes the Mexicans at the date of the Spanish conquest were living in an age of stone. It is true that they both knew and worked two metals with great facility, namely, gold and copper, especially the former; but gold is useless as a material from which to make implements, and copper was not sufficiently plentiful to be employed very freely. Both these metals were therefore utilized principally in the manufacture of ornaments, though copper tools were employed in wood-carving and a few minor arts. [Illustration: FIG. 19.--Stone and obsidian implements. 1, 2. Obsidian scrapers. 3–5. „ flakes. 6, 7. „ „ perhaps for inlay. 8. Chert knife-blade. 9. Obsidian core, from which flakes have been struck. (_British Museum_)] Hard stone from which adze- and axe-blades and chisels could be manufactured occurred in quantities in Mexico, and various volcanic rocks were quarried for the purpose of making implements. Chalcedony and other flinty materials were used in the manufacture of knife-blades, and the workmanship of these, especially of some of the specimens from the Mixtec country, are surpassed only by the finest examples of ripple-flaking from ancient Egypt (see Fig. 19, 8). Knife-blades of this form were usually thrust into a simple ball of resin to form a handle, though for ceremonial use elaborately carved wooden hafts covered with turquoise and shell mosaic (Pl. XVIII, 1) were fitted to them. But the material which perhaps was of the greatest use to the Mexicans was obsidian. From blocks of this natural glass long flakes could be detached by pressure, each of which came away from the core with a razor-edge. Such flakes were used as knives without further preparation, or were inset along the borders of wooden swords to form a cutting edge. The flaking-implement was a pointed stick with a cross-piece at one end; the artisan rested the point on a suitable spot on the obsidian block, which he held between his feet, and applied the necessary pressure by leaning his chest against the cross-piece. The nature of the long thin flakes which were detached, and of the core from which they were struck, is shown in Fig. 19, 3–5 and 9. Broader flakes were worked up by secondary flaking to form delicate arrow-heads and graceful spear-heads, while large slabs were polished to form mirrors, the surface being carefully finished by continuous rubbing with a handful of reeds. Masks were also made from this material, and details such as teeth and eyes were inlaid with shell and other substances. Stone axes (Fig. 20) were carefully ground by rubbing on a block of hard-stone, probably with sand or emery and water, the surface in the more highly-polished specimens being finished with bamboo. The shapes show some variation, but the implement usually tapers slightly towards the butt, which is rounded off more or less abruptly; some specimens are long-oval in transverse