The absolute clock utilized most widely in archaeology is the historical record. Men have used calendars for a long time, and have often left records with written dates. On tombstones at a site in old Virginia, on the pedestals of statues and other monuments from classical Greece and Rome, on the walls of the tombs of Egyptian kings, dates are clearly inscribed which can be related to the sites dug into by the archaeologists. These dates must often be recalculated in terms of the Christian calendar which we use. Most calendars in use in the Mediterranean, the Near East and China during classical antiquity have been successfully correlated with the one we use today, and a date inscribed or noted on such sites can be considered in our own terms. Other calendars, such as those developed in the ancient cultures of the Maya on the Yucatan Peninsula, have yet to be accurately correlated with our own. Such calendars can be used on their own terms of course, and a site which has an inscription in the Mayan calendar is known to be so many years older or younger than another one with a different date in that calendar. We speak of such a situation as a _floating chronology_. That is, the sequence of events and the number of years which separate them are known, but the dates of those events in absolute time are unknown. [Illustration: (uncaptioned)] Tree rings afford another kind of absolute clock, the _dendrological_ method. Each year a tree adds a growth ring. Depending on the amount of water the tree has available to it for cell growth, the ring will be wider or narrower. Certain trees whose water requirements are high live near streams or other places where their roots can tap a constant supply of water. Such trees, referred to as _complacent_, have annual rings which are all of about the same width. Other _sensitive_ trees live in places where they must depend almost wholly on rainfall for their water supply, as on the slopes of hills or in the clefts of rocks. Such trees have annual rings which vary in width depending upon the amount of rain they receive. In any given area, especially in arid and semiarid regions, some years have more rainfall than others. The sensitive trees will produce wider rings during years when there is more rainfall and narrower rings in years when there is less. Often there are periods of a decade or so when all of the sensitive trees will produce the same pattern of ring growth; for example, three years of narrow rings, one year of wide, two more of narrow and three more of wide. Such a pattern is called a _signature_.