In parts of France, some peasants still live in caves. In prehistoric time, many generations of people lived in them. As a result, many caves have deep layers of debris. The first people moved in and lived on the rock floor. They threw on the floor whatever they didn’t want, and they tracked in mud; nobody bothered to clean house in those days. Their debris--junk and mud and garbage and what not--became packed into a layer. As time went on, and generations passed, the layer grew thicker. Then there might have been a break in the occupation of the cave for a while. Perhaps the game animals got scarce and the people moved away; or maybe the cave became flooded. Later on, other people moved in and began making a new layer of their own on top of the first layer. Perhaps this process of layering went on in the same cave for a hundred thousand years; you can see what happened. The drawing on this page shows a section through such a cave. The earliest layer is on the bottom, the latest one on top. They go in order from bottom to top, earliest to latest. This is the _stratification_ we talked about (p. 12). [Illustration: SECTION OF SHELTER ON LOWER TERRACE, LE MOUSTIER] While we may find a mix-up in caves, it’s not nearly as bad as the mixing up that was done by glaciers. The animal bones and shells, the fireplaces, the bones of men, and the tools the men made all belong together, if they come from one layer. That’s the reason why the cave of Peking man is so important. It is also the reason why the caves in Europe and the Near East are so important. We can get an idea of which things belong together and which lot came earliest and which latest. In most cases, prehistoric men lived only in the mouths of caves. They didn’t like the dark inner chambers as places to live in. They preferred rock-shelters, at the bases of overhanging cliffs, if there was enough overhang to give shelter. When the weather was good, they no doubt lived in the open air as well. I’ll go on using the term “cave” since it’s more familiar, but remember that I really mean rock-shelter, as a place in which people actually lived. The most important European cave sites are in Spain, France, and central Europe; there are also sites in England and Italy. A few caves are known in the Near East and Africa, and no doubt more sites will be found when the out-of-the-way parts of Europe, Africa, and Asia are studied. AN “INDUSTRY” DEFINED