Signatures are the basis of tree ring chronologies. All the trees in a region did not begin growing at the same time of course, but every time there is a series of years which will produce a signature, all the sensitive trees still alive will have that signature. Let us say we cut down a sensitive tree in 1960 and, by counting back the rings, find signatures at 1940-45, 1910-14, 1880-89, 1821-27, 1795-1800 and 1750-58. Next we recover a beam from an abandoned Spanish Mission built in 1810. It happens to be from a sensitive tree, and we can spot the 1795-1800 and the 1750-58 signatures near the outer rings. Now we have two records which can be said to be _crossdated_. Let us assume that the Spanish Mission log give us signatures as far back as 1350. An abandoned Indian pueblo produces a log with signatures crossdating those of the Spanish Mission log and continuing the record back to 1250. Older and older archeological sites will yield older and older signatures with each log crossdating some of the signatures of younger logs. At present we have a tree ring calendar for certain species of trees extending back to about 100 B.C. This is called a _master tree ring chronology_. A log from an archaeological site may contain only one or two signatures and be, in itself, a floating chronology, but by comparison with the master chronology one can determine the cutting date. Having a series of cutting dates for the construction timbers in an archaeological site yields the probable dates at which the site was built and occupied. [Illustration: _Diagram to Illustrate Tree Ring Dating. Reproduced by special permission from Stallings, “Dating Prehistoric Ruins by Tree-Rings.”_] C THIS BEAM CAME FROM AN OLD HOUSE B THIS BEAM CAME FROM A HOUSE A THIS WAS A LIVING TREE WHEN CUT BY US THE RING PATTERNS MATCH AND OVERLAP BACK INTO TIME SPECIMENS TAKEN FROM RUINS, WHEN MATCHED AND OVERLAPPED AS INDICATED, PROGRESSIVELY EXTEND THE DATING BACK INTO PREHISTORIC TIMES. Not all kinds of trees can be used for dendrochronology. Pine, fir and pinyon are the most useful; juniper can sometimes be used. Oak, cottonwood, willow and others are very difficult to date and frequently cannot be used at all. Another widely used absolute clock is that based upon the orderly decomposition of carbon. This is the _radiocarbon_ or C-14 method of dating. Molecules of various substances are made up of atoms. We know now that not all atoms of a substance are precisely the same. We speak of _isotopes_ of an atom. To clarify this let us think of the atoms of carbon as being made up of a mass of ping-pong balls. Some atoms will have more ping-pong balls than others, but all will have enough and in the proper order to be carbon atoms. Each of the atoms with 14 ping-pong balls we shall refer to as the carbon-14 isotope of carbon. There will be other isotopes of carbon atoms too.